(BecomeUser) Implement "become user" functionality

Depends on:

* 3akai-ux: https://github.com/oaeproject/3akai-ux/pull/3737
* oae-rest: https://github.com/oaeproject/oae-rest/pull/2

Once this is approved, I'll have to change `package.json` to not refer to oae-rest on my branch.

There is no "unbecome" at this point. With some work we can probably implement that, however I'm not sure it is really necessary as opposed to just logging out.