Added title: gambit for discussions

[kevindierkx]

This adds support for a title: gambit for discussions.

If something is missing or incorrect let me know. ;)

[tobyzerner]

Thanks for the PR! 😄

Unfortunately I don't think we'll merge this one for two reasons:

1. Using LIKE in this case is not going to perform acceptably in large forums
2. We ultimately want to integrate title searching into the default keyword search, rather than as a gambit – see Search discussion titles too #257, and feel free to post there if you have any ideas!

Also, a heads up for future contributions: the copyright in the header comment block refers to the whole project rather than the individual file, so it shouldn't be changed.