Add CI config with code-style checking

Similar to briar-mailbox!3 (merged)

added Infrastructure label

added Sponsor 7 label

mentioned in issue #26 (closed)

marked this issue as related to #26 (closed)

I think we can do this similar to the mailbox: briar-mailbox@5c553e25

but probably best to not just add the .gitlab-ci.yml as in that revision, but use the latest one, I think there have been subsequent changes to it

assigned to @sebkur

added S: in progress label

mentioned in merge request !9 (merged)

added S: in code review label and removed S: in progress label

closed with merge request !9 (merged)

removed S: in code review label

changed milestone to %Desktop 0.1.0