Add support for merging accounts

[analogrelay]

When we don't properly auto-merge accounts, we should provide support for manual merging. All resources for the source user (reviews, comments, credentials, etc.) must be moved to the new user and the old user deleted.

The workflow for this is something like this:

1. Alice is logged in to her Facebook account and wants to merge with her Twitter account (which wasn't already merged).
2. Alice clicks "Merge with another account..." in the drop down for her name in the top-right corner.
3. The system displays the same OAuth prompt as it does when she logged in, but with the text "Select another provider to merge with"
4. She clicks one of the providers and completes authentication
5. The system merges the accounts and logs Alice in to the merged account.