This PR moves the "support is in #synapse:matrix.org" in the bug report template outside of the comment as some people seem to ignore what's in the comments, and phrase it a bit more like the support request template. It also adds a default issue template that says the same thing. It's also adding a notice about the security disclosure to both the default template and the bug report one.
It also adds a badge to the top of the README with an alt text saying about the same message if the badge doesn't load (e.g. if matrix.org is slow).
Fixes#6826
PR #2413 added an issue template, but just adding files to the project
directory upsets the packaging scripts: we need to explicitly include or
exclude them.
Move the template into a .github directory to make that easy, and to de-clutter
the root a bit.
