[BuildStream] The Marge-bot has landed!

[Laurence Urhegyi <laurence urhegyi codethink co uk>]

Hi,

Inspired by the freedesktop-sdk project, BuildStream now has a 
merge-bot, named marge-bot [0]. Marge is a gitlab user who will help us 
manage having lots of MRs being set to merge at the same time.

The way this works is: once you have the MR reviewed and approved to 
land, assign to Marge instead of hitting merge yourself, and she will 
handle the management of the current queue of MRs being set to merge. 
This means that if one pipeline succeeds and something is merged, other 
MRs are automatically rebased and set to merge, so you don't have to 
deal with this hassle.

Important: this now means that we should be assigning *all* MRs to be 
merged by the bot, since if we set some MRs to be merged manually it 
will interfere with what Marge is doing.

Cheers,
Laurence

[0] https://gitlab.com/marge-bot123