.gitlab: Provide issue templates

I would like to improve the bug reporting experience for Alpine users. Currently, we don't collect any metadata on reported issues and thus it is difficult to automatically assign these issues to package maintainers. Therefore, many issues are stale and it can take a long time before someone has a look at it. This can be frustrating for users. I would like to improve this situation by implementing the following:

1. Collect more metadata on bug reports through issues templates. This is implemented in this merge request.
2. Enhance alpine-qa-bot to parse issues and automatically assign certain issue types (package issues and package upgrade requests) to the maintainer of the affected package.
3. Periodically cleanup maintainer comments, i.e. remove inactive maintainers to ensure a swift response to created issues (similar to tsc#79). ¹

This pull request only implements the first step, I believe this to be useful independent of the other two steps. Feedback on the provided issus templates is more than welcome, I believe the three templates added here to be the most common issues kinds we have in our bug tracker at the moment. We can add additional templates in the future.

For testing purposes, I have enabled these templates in my fork. Feel free to create an issue there to experiment with the templates.

1. This is a long-term goal and probably warrants a TSC decision. ↩