Allow fetching MUC affiliations for non-admins

[nesium]

Opening this in favor of prose-im/prose-core-client#66

From what I understand, in order to request other affiliations you either need to be an admin, or if the room is "members-only" (i.e. not public) and the whois setting is "anyone" you only need to be a member.

I guess we have two options:

1. Modify the condition in the MUC module, or
2. Bump users from the same server to admin when they join a room, presumeably with the help of a custom module

Refs #8

[nesium]

Linking my custom module here as well: mod_muc_public_affiliations