Create team form fails silently when api call errors

There are database unique constraints on the osm-teams team.name and team.hashtag columns. To reproduce this issue, just go to /create-team and enter some values that already exist in the team table. The api returns a 400 error but the create team UI does not display any validation, it instead redirects to the teams index.

- [ ] make hashtag nullable (currently unique constraint prevents that)
- [ ] add client side validation to all create team form fields
- [ ] catch and display proper error message when API call fails, instead of redirecting to teams index page.