Prepare: run E2E smoketests with GitHub actions (#23301)

- Port E2E testing scripts from cypress-ui-automation
- Move server to docker-compose, move E2E images to ecrpublic
* Integrate General channel renaming, fixes
- Add local automation-dashboard
- Add readme
- Add E2E smoketests
- Bump postgres to 12

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Saturnino Abril <saturnino.abril@gmail.com>
Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

e2e-tests/.ci/server.stop.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -e -u -o pipefail
+cd $(dirname $0)
+. .e2erc
+
+mme2e_log "Stopping E2E containers"
+${MME2E_DC_SERVER} down -v