validate email uniqueness

[vilmibm]

currently people can sign up for an account with an email we already have on file. this is a bug and is currently caught manually. fail form validation for non-unique emails.

[MineRobber9000]

Where are emails stored? I could probably bang this out in a PR.

[vilmibm]

@MineRobber9000 They are stored in the Postgresql database via Django. The fix for this issue should be adding a unique=True to the email field in the Townie model and then creating migrations for the change.

[MineRobber9000]

What should I do with the migration? I know I should django-admin makemigrations (or however that tool is meant to be run) but what do I do with the resulting migration?

[vilmibm]

makemigrations should result in some new files. you should commit them and then you'll be good to go.