Add option to delete remote branch (after merge)

[mtsgrd]

Now that it is no longer necessary to auto-delete branches when merging stacks we should add an option for manually deleting the branch (after merge?).

Requested on discord: https://discord.com/channels/1060193121130000425/1073202153163857920/1324663015890485303

[ndom91]

We have some functionality which will automatically update the target branch of the next "lowest" branch in a stack after a user merges the one below it, if and only if they do not have the "auto delete branch after merging" option enabled in their repository. Because if that option is enabled, the merged branch is deleted and the target of any branch that used to point to it is updated.

So it should be easy enough to also offer the user an option to delete that branch upon merging if and only if they don't have the aforementioned option enabled in their repo too.

I've added this to my list to check out this week