Use Ragtime instead of Lobos

[venantius]

Lobos is weird and no longer in active development. Ragtime isn't great but it's the only reasonable alternative. Long live ragtime.

[venantius]

Doing this in stages:

• [x] write current migrations in ragtime
• [x] get tests passing
• [x] delete lobos
• [x] refactor github migrations into their own migrations file

replace team-users notion with "role" ? seems like a word that encapsulates the idea there.