web app: automatically highlight #Lxx lines

Add automatic line numbers from #Lxx location.hash

It supports #L50 and #L50-L150 (The latter will highlight from line 50 to line
150).

It also changes the hash if we click on a line, and if we click on a second
line holding the SHIFT key, it will highlight the whole range, and update the
hash as well.

Closes: #761103

Signed-off-by: Stefano Zacchiroli <zack@upsilon.cc>