Allow users to set their own, private, aliases on people they follow

[wlonk]

Per Joe:

maybe a little comment box we can fill in so we can remember that "mountainflyer25"="Jim Bob Walton".

These should be 100% private to the person who created them, and need a name other than "alias"; perhaps "mnemonic"?

This relates to #32 and #37.

[wlonk]

The FavStar component will now support adding a single "Nickname" to the Follow model, viewable by the from_user and applied when they look at the to_user. The interface will be something like a slide-out text input. You can update or blank out the nickname you set for a user, and it'll always fall back to their primary alias. You can also still see their underlying primary alias and username when you set a nickname. When you follow a user already, the FavStar will be a Bootstrap split-button with the option to change the nickname in the split dropdown.

Outstanding issue: how to update the displayed name elsewhere on the page when you change it in the FavStar.