Various bugfixes and improvements

Breaking Changes

• USE_PUBLIC_THREADS and USE_PRIVATE_THREADS permissions have been removed in favour of new threads permission
  • New permissions are split into CREATE_PUBLIC_THREADS, CREATE_PRIVATE_THREADS and SEND_MESSAGES_IN_THREADS (#799)
• GuildAvailableEvent will no longer fire when the bot joins new guilds
  • Some guild_create-ish methods were renamed to guild_available (#809)
• Remove hikari.errors.RESTErrorCode enum
  • The message that is sent with the error code is the info that the enum contained (#816)
• PermissionOverwrite doesn't inherit from Unique anymore and isn't hashable. Equality checks now consider all its fields. (#820)

Features

• Add new START_EMBEDDED_ACTIVITIES permission (#798)
• Support new channel_types field in CommandOption (#800)
• Add the add_component method to hikari.api.special_endpoints.ActionRowBuilder (#804)
• Add old_guild attribute to GuildLeaveEvent. (#806)
• Add GuildJoinEvent that will fire when the bot joins new guilds (#809)

Bugfixes

• Fix re-uploading forms with resources (#787)
• Prevent double linking embed resources, which causes them to upload twice
  • This was caused by attempting to move the resource from one embed to another (#788)
• Fix BulkDeleteError returning incorrect values for messages_skipped
  • This affected the __str__ and percentage_completion, which also returned incorrect values (#817)

Documentation Improvements

• Add docstrings to the remaining undocumented GatewayBot methods (#804)