New bug report and feature request templates

[kblaschke]

GitHub recently introduced a new way of setting up issue reporting forms, with more controls and the possibility to add additional instructions for bug reporters.

This PR replaces the existing two old Markdown templates with four new report forms:

1. A report form for bugs/crashes from projectM users, e.g. frontend-sdl2, the Qt frontend and possibly external (OSS) integrations.
2. A report form for issues affecting projectM developers, including people trying to build projectM code, working on projectM itself or integrating projectM into their own apps.
3. A feature request form which anyone can use to request features or tell us about new ideas regarding projectM or any related app.
4. A generic report form for anything else which doesn't fit in the above categories, e.g. preset discussions and other questions.

All report forms have a header which tells reporters that we won't accept any issues about the Android apps in the play store, with a checkbox that must be ticked where the user acknowledges this. This will surely not prevent people from ticking the box and creating an issue for the app anyway, but we can then close it without any further comment or discussion.

All other repos in the projectM org on GitHub will receive a config.yml in their issue template dir, which will disable the empty report link at the bottom and add a link that redirects to this repo here. We can then triage new reports and move them to the correct repository, without requiring the reporter to check in advance where the issue actually belongs.