Moved signup form embed to the membership settings

refs https://github.com/TryGhost/Team/issues/3340

ghost/admin/app/components/settings/newsletters.hbs

@@ -142,8 +142,4 @@
             {{/unless}}
         </div>
     </section>
-
-    {{#if (feature "signupForm")}}
-        <Settings::SignupFormEmbed />
-    {{/if}}
 {{/if}}

ghost/admin/app/components/settings/signup-form-embed.hbs

@@ -4,7 +4,7 @@
         <div class="gh-expandable-block">
             <div class="gh-expandable-header">
                 <div>
-                    <h4 class="gh-expandable-title">Embeddable Signup Form</h4>
+                    <h4 class="gh-expandable-title">Embed a signup form</h4>
                     <p class="gh-expandable-description">Grow your audience from anywhere on the web</p>
                 </div>
                 <button type="button" class="gh-btn" {{on "click" this.open}}>

ghost/admin/app/templates/settings/membership.hbs

@@ -166,6 +166,10 @@
         </div>
     </section>
 
+    {{#if (feature "signupForm")}}
+        <Settings::SignupFormEmbed />
+    {{/if}}
+
     {{#if this.showPortalSettings}}
         <GhFullscreenModal
             @modal="portal-settings"