Fix user settings page for Firefox

References #4050

- Restructures the CSS to work in Firefox - No visual changes, only fixes
Let Ghost serve favicon instead of using dependency

no ref
- Remove static-favicon dependency
- Refactor robots.txt middleware to also serve favicon
- Add ETag
Cleanup reserved words in generateSlug

closes #1889
- Adds a new filter for slugs
- Move slugs to config.slugs
Properly handle no posts

fixes #4058
- Set noPosts in setupController
Merge pull request #3992 from halfdan/favicon

Let Ghost serve favicon instead of using dependency
Merge pull request #4060 from halfdan/4058-no-posts

Properly handle no posts
Merge pull request #4051 from PaulAdamDavis/fix-user-settings

Fix user settings page for Firefox
Merge pull request #3976 from halfdan/1889-generate-slug

Cleanup reserved words in generateSlug
Merge branch 'pr/3876'

* pr/3876:
  Redirect user if signup invitation isn't valid