Same as #232 but squashed (Implement MSC4144)

[SwirlyStone5877]

had to make a new mr because git squashing is a mess

relates to #208

[SwirlyStone5877]

why does github keep saying this branch can't be rebased... grah

[SwirlyStone5877]

i should explain the rationale behind the new ID: msg.UserID + "/" + username.plain,
matterbridge has ExtractNicks as a feature. unfortunately this doesn't change the user id of the recieved message, which could make some supporting clients think that two people are the same person, which is not true in reality
this did happen when i tried setting up a mcgalaxy-irc bridge and tried bridging that to matrix with this feature
see this screenshot:

i don't have a screenshot of the irc side on me, but the first message in the ss was sent by the system on the mc bridge while the rest were sent by me; twice on the mc bridge, once on my own irc account, then thrice more on the mc bridge