Specify default permissions for '_everyone' and '_users'

[bates64]

We currently don't specify what permissions servers should grant to _everyone and _users with no extra configuration, and what they should never grant to _everyone regardless.

For example, _everyone should never be allowed to sendMessages.

(CC @towerofnix as this may or may not be implemented in @decent/server?)

[towerofnix]

Here's the permission table used in @decent/server.

	_everyone	_guest	_user
manageChannels	False	-	-
manageEmotes	False	-	-
managePins	False	-	-
manageRoles	False	-	-
manageServer	False	-	-
manageUsers	False	-	-
readMessages	True	-	-
sendMessages	False	-	True
sendSystemMessages	False	-	-
uploadImages	False	-	-
The _guest and _everyone roles can ONLY have the following permissions set to true: readMessages.
The Owner role is created with ALL of the above set to true.

(allowNonUnique isn't inside that table because it isn't reeeeeally defined to do anything yet, but I expect that it would be settable on _everyone, for the purpose of signing up with a username that has already been used before. But I think allowNonUnique as a single permission is silly and that whether or not usernames have to be unique should be a server setting; anyways, that's for a different issue :) )