Auto-tag releases

[ErinCall]

I've been trying to follow the recommended versioning process for go modules. The auto_tag setting in our .drone.yml ensures that the docker image gets the version number from the git tag, so that part is handled.

However, the current process for creating version tags is "after merging a pull request, remember to put a tag on the merge commit, then remember to push the tag." That's two "remember to"s too many. We should find something that can increment the version number automatically.

I haven't found a drone plugin that can do it for us, but I did find a github Action plugin that should be pretty easy to port to drone (it might work right out of the box, if the image is published somewhere). It looks for #major, #minor, or #patch in any commit message since the previous tag and bumps the version accordingly. It can optionally do a patchlevel bump if nothing else is specified.