This implements similar behavior to sytest where a matching branch is used,
if one exists. This is useful when needing to modify both application code
and tests at the same time. The following rules are used to find a matching
complement branch:

1. Search for the branch name of the pull request. (E.g. feature/foo.)
2. Search for the base branch of the pull request. (E.g. develop or release-vX.Y.)
3. Search for the reference branch of the commit. (E.g. master or release-vX.Y.)
4. Fallback to 'master', the default complement branch name.

This PR updates the build tags that we perform Complement runs with to match our [buildkite pipeline](https://github.com/matrix-org/pipelines/blob/618b3e90bcae8efd1a71502ae95b7913e6e24665/synapse/pipeline.yml#L570), as well as adding `msc2403` (as it will be required once #9359 is merged). Build tags are what we use to determine which tests to run in Complement (really it determines which test files are compiled into the final binary).

I haven't put in a comment about updating the buildkite side here, as we've decided to migrate fully to GitHub Actions anyhow.

... using the script from matrix-org/sytest#1052

* Fix GitHub Actions lint for newsfragments

Signed-off-by: Dan Callahan <danc@element.io>

... and test it.

Signed-off-by: Dan Callahan <danc@element.io>

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 

* Add lint dependencies black, flake8 and isort

These are required when running the `lint.sh` dev scripts.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Add contributer docs for using the providers linters script

Add also to the pull request template to avoid build failures due
to people not knowing that linters need running.

Signed-off-by: Jason Robinson <jasonr@matrix.org>

* Fix mention of linter errors correction

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Add mention for installing linter dependencies

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Remove linters from python dependencies as per PR review

Signed-off-by: Jason Robinson <jasonr@matrix.org>

cf #4878

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.