Add merge_stable action to dlang-bot

[wilzbach]

Whenever a new commit gets pushed to stable, dlang-bot should submit a PR to master (if it can without merge conflicts).

• clone the repository
• execute sth. like https://github.com/wilzbach/git-tools/blob/master/merge_stable

If there are merge conflicts, it should add a comment to the merged PR.

[MartinNowak]

Apparently not possible to have empty PRs on GH :/. But we could always create (if not exists) a PR on pushes to stable. Even better with auto-merge already applied (once #69 is fixed).

[wilzbach]

See also: http://forum.dlang.org/post/mailman.2275.1516078491.9493.dlang-internal@puremagic.com

[MartinNowak]

We could submit a PR even when there are merge conflicts, and comment on that one to ask for merge conflict resolution.
Ideally we'd have a bit of a delay, e.g. 1hour after a push to stable, so we don't trigger CI with 10 individual commits.
Also I still need to manually merge stable to master to update the website during releases. Automation in that area has been fairly hindering and the dlang.org CI needs manual review atm. anyhow. Maybe better to exempt dlang.org from this automation.