Deletion of aliases

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

A user should be able to delete one of their aliases.

Describe the solution you'd like

A button/endpoint that allows for deletion. Since we will want usernames to be single-use to prevent squatting, the page should include sufficient warnings about what will happen.

Open questions

1. Do we want to rate limit creation/deletion to prevent DOSing the tombstone list?
2. Do we want to allow changing a username/alias so bio/extras/etc. transfer too?

Additional context

Depends on #648