Feat/user channel junction

[oskarrough]

Wait for #22 to be merged first.

• adds a new user_channel table
• creates the row in the db from the front-end when you create a channel
• is automatically deleted (by postgres) with the channel
• created_at and updated_at are now set automatically with Postgres

I did not touch any custom "roles" or such and there are some supabase-supported "auth.role" thing available as well https://supabase.io/docs/learn/auth-deep-dive/auth-row-level-security

Closes #7 Closes #20