Fix brakeman warning (#15870)

Fix brakeman warning (#15870)

As far as I understand, the brakeman warning was a false-positive as
`content_tag` properly escapes untrusted HTML. Furthermore, the interpolated
string values are built from the “username” part of accounts, which is
restricted to a small subset of ASCII that precludes any XML entity or HTML
code.

This proposed change should be functionally equivalent to the current code,
however it is slightly more robust, it's more idiomatic, and Brakeman will
stop complaining about it.