Cherry-picking policy

[Vlad Zahorodnii]

Hi,

At the moment, we have the following bugfixing workflow in plasma:

* fix a bug in master
* cherry pick the fix to stable branch(es)

The last step is usually done without creating a MR. If there is a merge 
conflict, some people do create a MR though.

I propose to make creating MRs for bugfix backports mandatory:

- this helps us to ensure that CI is happy
- increases the visibility of what happens in stable branches. For 
example some people help with backporting bugfixes, but they can forget 
to include a commit if the original MR included multiple commits. 
Hopefully, this can help us to catch these cases

Ideally this should be automated, but we have no tooling to do that.

Thoughts?

Regards,
Vlad