nick handling of '@freenode_Pangolin`:matrix.org' fails due to the backtick

[sbts]

The result is the backtick commences an inline code block, meaning the simple form of the nick

Pangolin`:

becomes
[Pangolin](https://matrix.to/#/@freenode\_Pangolin:matrix.org):

as displayed once the event is posted

This can be reproduced easily enough by clicking on, or tab completing a nick containing a single backtick

Specifically this is seen running Chrome on Linux
against riot.im/develop

[turt2live]

Similar, if not duplicate of, #4867

[sbts]

@turt2live Yep, looks like it's fundamentally a duplicate.
Although the original report was related to a trailing backslash, which was fixed.
As #4867 is still open, I'll close this one