[Permissions] Modify default level

# Feature request

#### Select the type of feature you are requesting:

- [ ] Cog
- [X] Command
- [X] API functionality

#### Describe your requested feature

Copied from original issue (#3087):

Provide a command to allow changing the default permission level of checks

That is to say, a bot owner (And nobody else due to the larger implications of this) should be able to swap the default permission level from checking against

none <-> mod <-> admin <-> guildowner

This should not allow modifying owner only checks still.

This would require an additional field in the permissions acl format, an exposed method to modify this in commands.Requires and a command for setting directly.

Start by tracing the permissions ACL format and the Commands.Requires functionality mentioned in the issue. Determine how the default level is represented and how a bot-owner-only command would update it. Done means supporting the none, mod, admin, and guildowner defaults without changing owner-only checks, with command and API access covered.

Written by the indexing model from the issue text.