MDL-70276 github actions: First cut, phpunit and grunt checks

MDL-70276 github actions: First cut, phpunit and grunt checks

First working version, supports phpunit (using build matrix):

- php72 (lowest), running mysql.
- php74 (highest), running postgres.

Also verifies that the branch has been "gruntified" and there
isn't any missing change (build js/css files).

TODO: Verify the remaining checks currently in .travis.yml, namely:
 - CITEST
 - Add caching
 - Better health-check for DB images.
 - Support from the tracker (satus badges and enable check).
 - Support from CiBoT (status and enable check).
 - Consider moving both the common setup (git, composer...)
   and the database (mysql, postgres) to own actions for
   easier tweaking.