Koa.3 and Lilac.master

Steps for open-release/koa.3 tag:

@Ned Batchelder (Deactivated) apply the recent Django 2.2.20 security patch to koa.master
@Ned Batchelder (Deactivated) add Adolfo to open-release team
@Former user (Deleted) makes sure there are no relevant incomplete tickets with the koa.3 milestone
@Former user (Deleted) make the k.3rc1.1 branch on koa.master
@Former user (Deleted) test the rc
make the koa.3 tag
ping Régis
update the relevant doc pages (make PR)

Steps for open-release/lilac.master branch

check the set of repos for Lilac, and keep open-release team up to date
make the lilac.master branch
update the RELEASE_LINE constant
update the transifex configuration
update Django to 2.2.20 (it’s a mix of 17, 19, and 20 now)
update the docs for the lilac branch
update the edx-platform and configuration pull request templates to mention lilac.
announce lilac.master inside edX. Mention the PR template change.
remove Adolfo from open-release team
announce the lilac.master branch within BTR. Explain that it’s only for testing, not production.