Correct logout URLs. Fixes #646
First steps towards suggestions.

Tackling video suggestions first, since they seem the highest leverage for the site.

Suggestion model: Intended to be a generic storage object for suggestions about anything.

Todo:
- Suggestion accepter classes, that consume a Suggestion and apply it to the appropriate model.
- Suggestion validation: don't accept suggestions for things we already know
Adds admin review tool for Suggestions.

Reworked some of how we store data to allow for negative reviews

Todo
- Write class to actually accept suggestions
Allow accepting suggestions.
Test for MatchSuggestionAccepter.
Change button color for suggestions.
Improvements to the flow for suggesting videos.

- Admin pages have thumbnail video previews.
- Redirect users to the same page after logging in
- Log out links for admins and users
- Slightly bigger labels in admin tool
- Easier youtube video searching buttons for adding page
Improvements to the styling of the pending suggestions.
Fix problem when not accepting any suggestions.
Display success banner on video suggestion.
Add additional sig image files. Fixes #647