github: modernise issue and pull request templates

Convert the bug report and feature request templates to GitHub issue forms.

Rewrite the pull request template to require that non-trivial changes are
discussed in an issue first, and to spell out that backend changes need a
clean test_all run and a test account for the integration tester before
they can be merged. Document the latter requirement in CONTRIBUTING.md.

Remove the legacy ISSUE_TEMPLATE.md which is superseded by the forms.

CONTRIBUTING.md

@@ -623,6 +623,15 @@ Or if you want to run the integration tests manually:
 - If your remote defines `ListR` check with this also
   - `go test -v -remote TestRemote: -fast-list`
 
+Before a new or changed backend can be merged we require:
+
+- A clean run of `go run ./fstest/test_all -backends remote` (please include the
+  result in your pull request).
+- A test account for the backend so the maintainers can add it to the
+  [integration test server](https://integration.rclone.org) and keep the backend
+  working as rclone evolves. A backend that we cannot test is likely to break and
+  may be removed.
+
 See the [testing](#testing) section for more information on integration tests.
 
 ### Backend documentation