Add Issues and PullRequest templates

As mentioned in this page: https://github.com/postmarketOS/pmbootstrap/community they suggest to add templates to the new Issues and PullRequests. I've noticed that some @ollieparanoid 's Pull Requests are very structured and it also simplify reviewing and testing them.

Something like:

Introduction
Problem
Proposed solution
Changes
Testing
Fixes #(number)

What you think?

For new issues it will be a little bit harder, since we have various types (issues, help requests, new features,...). I've no idea what we can use as template.

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