Update several links to allow the remote to use its configured default
branch name, rather than specifying `master` as the default branch name.
This will help avoid breakage if any of these projects rename their
default branch in the future.
Fix a few of the links where they were hitting redirects or had moved.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #2348
This also gives details of how to report a security issue, including the
key point that merge requests are (unfortunately) not confidential.
Heavily based on the flatpak security policy which just landed:
https://github.com/flatpak/flatpak/blob/master/SECURITY.md
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>