link: minimal initial view result

On initial load, instead of getting the first 25 links for every
collection, only get their totals and unread counts. This drastically
reduces the pageload time.

Updates the frontend to match:
- Handle initial results without link content.
- Dynamically load in collection's links, even for page 0.

pkg/arvo/app/link-view.hoon

@@ -1,13 +1,13 @@
 ::  link-view: frontend endpoints
 ::
 ::  endpoints, mapping onto link-store's paths. p is for page as in pagination.
-::  updates only work for page 0.
+::  only the /0/submissions endpoint provides updates.
 ::  as with link-store, urls are expected to use +wood encoding.
 ::
-::    /json/[p]/submissions                           pages for all groups
-::    /json/[p]/submissions/[some-group]              page for one group
-::    /json/[p]/discussions/[wood-url]/[some-group]   page for url in group
-::    /json/[n]/submission/[wood-url]/[some-group]    nth matching submission
+::    /json/0/submissions                             initial + updates for all
+::    /json/[p]/submissions/[collection]              page for one collection
+::    /json/[p]/discussions/[wood-url]/[collection]   page for url in collection
+::    /json/[n]/submission/[wood-url]/[collection]    nth matching submission
 ::    /json/seen                                      mark-as-read updates
 ::
 /-  *link-view,
@@ -491,9 +491,11 @@
 ::
 ++  give-initial-submissions
   ~/  %link-view-initial-submissions
-  |=  [p=@ud =path]
+  |=  [p=@ud =requested=path]
   ^-  (list card)
-  :_  ?:  =(0 p)  ~
+  :_  ::  only keep the base case alive (for updates), kick all others
+      ::
+      ?:  &(=(0 p) ?=(~ requested-path))  ~
       [%give %kick ~ ~]~
   =;  =json
     [%give %fact ~ %json !>(json)]
@@ -501,9 +503,9 @@
   %-  pairs:enjs:format
   %+  turn
     %~  tap  by
-    %+  scry-for  (map ^path submissions)
-    [%submissions path]
-  |=  [=^path =submissions]
+    %+  scry-for  (map path submissions)
+    [%submissions requested-path]
+  |=  [=path =submissions]
   ^-  [@t json]
   :-  (spat path)
   =;  =json
@@ -516,6 +518,15 @@
     %~  wyt  in
     %+  scry-for  (set url)
     [%unseen path]
+  ?:  &(=(0 p) ?=(~ requested-path))
+    ::  for a broad-scope initial result, only give total counts
+    ::
+    =,  enjs:format
+    %-  pairs
+    =+  l=(lent submissions)
+    :~  'totalItems'^(numb l)
+        'totalPages'^(numb (div l page-size))
+    ==
   %^  page-to-json  p
     %+  get-paginated  `p
     submissions

pkg/interface/link/src/js/components/links-list.js

@@ -19,11 +19,18 @@ export class Links extends Component {
     this.componentDidUpdate();
   }
 
-  componentDidUpdate() {
+  componentDidUpdate(prevProps) {
     const linkPage = this.props.page;
-    if ( (this.props.page != 0) &&
-         (!this.props.links[linkPage] ||
-          this.props.links.local[linkPage])
+    // if we just navigated to this particular page,
+    // and don't have links for it yet,
+    // or the links we have might not be complete,
+    // request the links for that page.
+    if ( (!prevProps ||
+          linkPage !== prevProps.page ||
+          this.props.resourcePath !== prevProps.resourcePath
+         ) &&
+         !this.props.links[linkPage] ||
+         this.props.links.local[linkPage]
     ) {
       api.getPage(this.props.resourcePath, this.props.page);
     }

pkg/interface/link/src/js/reducers/link-update.js

@@ -37,8 +37,10 @@ export class LinkUpdateReducer {
 
         // since data contains an up-to-date full version of the page,
         // we can safely overwrite the one in state.
-        state.links[path][page] = here.page;
-        state.links[path].local[page] = false;
+        if (typeof page === 'number' && here.page) {
+          state.links[path][page] = here.page;
+          state.links[path].local[page] = false;
+        }
         state.links[path].totalPages = here.totalPages;
         state.links[path].totalItems = here.totalItems;
         state.links[path].unseenCount = here.unseenCount;
@@ -48,7 +50,7 @@ export class LinkUpdateReducer {
         if (!state.seen[path]) {
           state.seen[path] = {};
         }
-        here.page.map(submission => {
+        (here.page || []).map(submission => {
           state.seen[path][submission.url] = submission.seen;
         });
       }