fixes#10990
- Changed the static router to throw a 400 error for a missing template file, rather than falling back to using the default.hbs file
- Falling back is weird and hard to understand, but throwing an error makes it clear that the user has to provide the matching template
- The new error reads 'Missing template [filename].hbs for route "[route]".'
Assume you have a route.yaml file something like:
```
routes:
/: home
```
- In Ghost v2, if you don't have a home.hbs template, Ghost falls back to using the default.hbs file if it's available
- Most themes have a default.hbs, however this file is a layout file, depended on by other templates, not a template file itself
- In production mode, using the default.hbs as a template causes weird, intermittent layout issues depending on which order pages are loaded
- This is due to this issue: https://github.com/barc/express-hbs/issues/161
- In Ghost v3, we will throw a 400 error for missing template files instead of having a fallback
- In the example above, navigating to '/' would throw the error 'Missing template home.hbs for route "/".'
refs #10790
- Moved /core/apps into core/frontend
- Moved /core/server/helpers to /core/frontend/helpers along with /core/server/services/themes
- Changed helper location in overrides
- Moved /core/server/services/routing to /core/frontend/services
- Moved /core/server/services/url to /core/frontend/services
- Moved /core/server/data/meta to /core/frontend/meta
- Moved /core/server/services/rss to /core/frontend/services
- Moved /core/server/data/xml to /core/frontend/services
