[dreamwidth/dw-free] 17bad4: Implement @foo user mentions

[github]

Branch: refs/heads/develop
Home: https://github.com/dreamwidth/dw-free
Commit: 17bad4534c87fe74f64c774cf13152e187451fe8
https://github.com/dreamwidth/dw-free/commit/17bad4534c87fe74f64c774cf13152e187451fe8
Author: Mark Smith <mark@dreamwidth.org>
Date: 2019-06-24 (Mon, 24 Jun 2019)

Changed paths:
M cgi-bin/LJ/CleanHTML.pm

Log Message:
-----------
Implement foo user mentions

As per suggestion: https://dw-suggestions.dreamwidth.org/1527807.html

This makes the usage "foo" and "[Bad username or site: foo @ BAR]" work everywhere, where BAR is
one of the external sites. Such as "foo" or "foo". This is
what we already do in Markdown so it's easy to expand it to the rest of
our HTML cleaning.