Activity refactor

- Refactored the activity template. Dramatically reduced the amount of repeated HTML
- Re-use existing macros where possible
- Added metadata to activity tiles
- Made the activity widget properly internationalizable in favor of string concatenation + support a wider variety of summary messages
- Fixed the logic behind when to show the "Show all comments" link
- Upgraded to the latest version of underscore
- All l10n functions now take the user's timezone into account
- Internationalized the timeago plugin
- Added l10n wrapper for timeago