[API] Bulk bans and ban list subscriptions

Describe the solution you'd like
The ban list returned by get guild bans API should have a list ID on top of current fields (ban list ID would be represented as the ID of the ban entry on user zero, for format compatibility with clients for discord.com; and the reason field of the ban entry of user ID zero would be used as the title of the ban list), which we could use in our ban lists to import that ban list as a whole. Each ban in the ban list should also have its own ban ID too (this would be handy for ability to edit the reason of a particular ban).

Then, using a PUT /guilds/{guild.id}/bans/{banlist.id} would add whole ban list with the specified ID into that guild's ban list by reference. Removing the reference to a banlist from the guild's banlist would be done similarly to removing any individual ban from the banlist.

TODO: how to adapt this into DMs, group DMs and user groups

No implementation files or tests are named. Start by locating the existing get guild bans API and the ban-removal route, then define how list and individual ban IDs, references, and titles should be represented. Resolve the open questions for DMs, group DMs, and user groups before implementation; done means the API and reference behavior are specified and covered by tests.