Add a release candidate template and support creation in release/tasks

Part of https://gitlab.com/gitlab-org/release-tools/issues/168

Example run:

release-tools rs-rc-tasks % CI=true be rake 'patch_issue[10.6.0-rc6]'
--> Issue "Release 10.6.0-rc6" created.
    https://gitlab.com/gitlab-org/release/tasks/issues/129

And the resulting issue: https://gitlab.com/gitlab-org/release/tasks/issues/129 🎉

Interested to hear thoughts on the new formatting from @marin, @jameslopez, @rymai, and anyone else.