Why this change?
As the number of themes which the Discourse team supports officially
grows, we want to ensure that changes made to Discourse core do not
break the plugins. As such, we are adding a step to our Github actions
test job to run the system tests for all official themes.
What does this change do?
This change adds a step to our Github actions test job to run the system
tests for all official plugins. This is achieved by the introduction of
the `themes:install_all_official` Rake task which installs all the
themes that are officially supported by the Discourse team.
