Display/use user names instead of IDs for user management

[jagoe]

Currently, the commands /addusers, /deleteusers and /listusers only work with IDs as both input and output.
In the context of a chat group, BabelBot should be able to work with user names/user references instead of IDs, improving control over who is actually able to use BabelBot.

[jagoe]

@RauchF This is another issue that might distinguish between different contexts. Maybe we should think about storing permissions per group or limiting BabelBot to a certain number of groups?

[RauchF]

Maybe we could start with the "canonical" user names - we could see whether the arguments match something like @(\w+) and in that case try to look them up - that should work whether used in a group or in a DM with the bot.

In general, I don't mind storing permissions per group. What does come to mind though is whether those users should still be able to direct message BabelBot. In case of i.e. some "news"-like channel I'd definitely see the use in only letting users use it inside that group. For teams that have individual agents roaming around that would be a disadvantage though. So I think it should probably be configurable if we go that way.

If we were to store permissions per group, as long as we don't store a ton of information for a group until there's actually something to store, I don't think we'd really need to limit the number of groups.