Add handler helpers

[PaulSonOfLars]

What

Add new helper methods to make it easier to enable channels or edited messages on commands.

Unsure about this PR as it stands; I notice these things:

• the AllowX fields are inconsistent; why doesnt the ChatMember handler have an AllowChannel field? or the ChatJoinRequest handlers?
• I can't really remove them, as this would break existing bots in weird ways ("Why does my bot now do stuff in channels?")

Impact

• Are your changes backwards compatible? Y
• Have you included documentation, or updated existing documentation? Y
• Do errors and log messages provide enough context? Y