Sync

Merging the branch sync with master, here are some changes:
- Store posts from followed channels
- Channels on sidebar sorted as described on: https://buddycloud.org/wiki/Buddycloud_client_design#buddycloud_sorts_the_most_interesting_information_for_you

Some current bugs on master should also be fixed:
- Sidebar scrollbar not appearing
- Posts sorting
- Unable to follow/unfollow channels

It's also expected some issues, mainly for the user first ever login if the user follows a lot of active channels, the /sync API call is a bit slow and the browser needs to store all posts before rendering, what takes a while.