Most of the changes required for the dark theme

The only thing that remains is the Reddit icon. Waiting on @NSDex to
provide a version with a transparent background, and an inverted
version.
Increase specificity of Twitter selector

Also, looks like I forgot a few changes in the last commit.
Add links to browser extensions on home page
Fix issue with exiting fullscreen on webkit

Closes #297
And again for Firefox
Merge pull request #296 from MediaCrush/extensions

Add links to browser extensions on home page
Merge branch 'master' into dark-theme
Add inverted reddit icon
Merge branch 'master' into dark-theme
Merge pull request #298 from MediaCrush/dark-theme

Add dark theme
Modify return codes from the API per #299

This is a **breaking** change.
Modify dark theme to affect view pages as well
Update pagination on /mine for dark theme
Merge branch 'master' into embed-improvements