Allow bug_link_tag prefix in jump to bug form

Until now, due to the use of gpc_get_int(), an error was thrown if the
user entered #1234 in the search field, which sometimes happen when
copy/pasting e.g from a Git commit message.

As convenience, we now ignore the leading # (technically, the value of
$g_bug_link_tag config) and extract the bug id from the rest of the
string.

Fixes 0037134