- Remove the `plugins.remove` API method.
- Throw an error when attempting to register multiple plugins with the same name.

updates #752

To illustrate that this is not necessary (and also not possible with
non-bundled plugins).

to indicate that it's private and to differentiate it from the outer "global"
converse object.

Specifically involving querying by the 'xmlns' attribute, where I've had issues
in the past.

Prosody doesn't handle it if we set the affiliation for multiple JIDs in one
stanza, so we need to send a stanza for each JID.

Refs https://prosody.im/issues/issue/795

Apparently the mailing list hasn't been working since months.

Now that we are sending multiple IQ stanzas when setting affiliations.
We now again have a sendAffiliation method, so we use that.

At least for Prosody.

This changed required me to go back to an array of maps instead of a single map
to store jids and affiliations.

When inviting someone to a members-only room, you only want to add an
affiliation of 'member' if they don't already have any other affiliation.

So we need to ignore the "new" member if they're already on *any* of the lists.

Make the delta algorithm simpler. Fixed some delta bugs in the process.

One for each affiliation (member, owner, admin).

Unfortunately Prosody doesn't give you correct results if you query for all
three affiliations in one stanza.

We'll have to see whether setting all three in one stanza actually works.