Update index page to be a central hub for snippets

[benjaminheng]

After logging in, the index page (/) will be a central hub showing snippets the user has permission to view.

In this page, the user can select from a dropdown to view:

• their own snippets
• snippets from particular groups
• snippets from people they're following

Additional controls are available, such as:

• Search snippets from the currently displayed list
• Sort snippets by created/updated date

[benjaminheng]

Pending: searches