[#380] Resolve "eliminate channel names and descriptions"

Closes #380 (closed)

NOTE: leaving WIP until we have given users 24 hours notice of this (disruptive!) change!

context

• channel names and descriptions are helpful, but they leave potentially compromising metadata in persistent cleartext on both signalboost's servers and the devices of our users
• this MR offers changes to (1) eliminate descriptions entirely, (2) set names to 2-word diceware phrases that admins cannot change

changes

provide fixed/random names:

• add backfill migration to overwrite names of all channels w/ 2-word diceware phrases (using niceware lib)
• remove user-provided name from channel creation logic. instead create a 2-word diceware name in channelRegistrar.create
• remove all RENAME logic from application

eliminate description:

• remove channel.description from data model
• remove all usages of chan description from application layer

side-effects:

• fix bug in bin/dev/migrate-undo (didn't appropriately specify base dockerfile)