implement dashboard / homepage

[JonathanReeve]

This should have a few steps, which I'll break out into separate issues:

• [x] disable excerpts on activity items (#311)
• [x] make a profile widget with the logged-in user's avatar, profile completeness, and links to other personalized pages (#314)
• [x] make filter tabs that filter the activity stream with AJAX (#313)
• [ ] make a new sidebar called "tabbed" that displays all its widgets as tabs (#315)
• [x] add news rss, sites, resources as widgets to tabbed sidebar
• [x] write script to add the widgets to new sidebars, to automate the rollout process.

[terrainsvagues]

The homepage should display (regardless of logged in/logged out status): 3 most recent blog posts, 3 most recent CORE deposits, 3 most recent forum posts