github: move ISSUE_TEMPLATE.md to ISSUE_TEMPLATE/bug_report.md

GitHub supports different types of issue templates for a while now.
The following types are supported:

* Bug report
  (living in `.github/ISSUE_TEMPLATE/bug_report.md`),
* Feature request
  (living in `.github/ISSUE_TEMPLATE/feature_request.md`), and
* Custom issue template
  (living in `.github/ISSUE_TEMPLATE/custom.md`)

Our current issue template is mostly a bug report template, so this
moves it to `.github/ISSUE_TEMPLATE/bug_report.md`.