Add a sponsor button with links to matrixdotorg's patreon and liberapay accounts.

Inserts a zero-width space in the `-->` which isn't supposed to close a
comment. This used to be here but it got lost in
d8682627.

* Update PR template to use absolute links

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Signed-off-by: Aaron Raimist <aaron@raim.ist>

https://help.github.com/articles/adding-support-resources-to-your-project/

Signed-off-by: Aaron Raimist <aaron@raim.ist>

request backticks for logs

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.