render html in notifications

fixes #2844
Added error link on 404 pages

closes #2298
- added hyperlink to error templates
- added class for styling hyperlinks on error pages
Merge pull request #2792 from jomahoney/2298-error-page

Added error link on 404 pages
Merge pull request #2854 from iBotPeaches/render_notifications

render html in notifications
creating new (ajax friendly) end-point for signing out
initial implementation of signing out from the new Ember UI