Merge Proposals need a title, so do MP emails

Merge Proposals have an (optional) description but don't have a title.

As a result emails notifying MPs and MPs headers themselves are very cryptic: Even when you are an owner of the code being modified you need to read several lines in the email or webpage till you can figure out what the MP is about. Usually you either find the description (if you are lucky and there is one) or have to check the diffs themselves.

If MPs had a COMPULSORY title, it could be used both at the very top of the MP webpage and in the MP notification email subject.

That would allow people to know what the MP is about at a glance.

Title size could follow similar rules to git commit title/first line conventions.

Other similar tools [1] use the title field in this way.
[1] https://www.reviewboard.org/