New feature: Associate a Twitter username with a maker or project if there isn't one

[ginatrapani]

Sometimes people miss the Twitter autofills, or the right one doesn't show up in search results. After a maker or project has been created, there should be some way to associate a Twitter account with someone (and eventually, a GitHub account, a Dribbble account, etc).

This action should also update the users table to link the two; and enforce the unique autofill from #241