the rate_limit_per_user field has been on schema.CHAN_UPDATE for a while
but the database didn't have the column.
- migration.scripts: add 7_text_channels_rate_limit_per_user.sql
- schema.sql: add guild_text_channels.rate_limit_per_user
All icons will be invalidated.
- images: change icon path model
- images: handle hashes being NULL, for emojis only needing
the key
- schemas: add NEW_EMOJI, PATCH_EMOJI
- migration.scripts: add 3_drop_constraints_icons_hash
- schema.sql: drop unique and not null from hash, change primary key in
icons