Add UI for managing Sync/FxA

[grigoryk]

Depends on #206. Related: #201.

In our current browsers, we have UI that allows configuring various sync-related functionality.

For a first pass, we're primarily concerned with allowing user to select what should be synced and what shouldn't be. This could be a simple pop-up multi-selection tool, or we could introduce a new section to Settings, which would also allow managing some aspects of the Firefox Account.

[grigoryk]

We already have this!