Decouple repos from mediawiki gate queue
A rather long change but it is covered by tests. Create a bunch of jobs
prefixed with 'mwgate-':
mwgate-composer-hhvm-jessie mwgate-composer-php55-trusty mwgate-composer-php70-jessie mwgate-composer-validate mwgate-jsduck mwgate-jshint mwgate-jsonlint mwgate-npm-node-6-jessie mwgate-php55lint mwgate-rake-jessie mwgate-tox-jessie
Adjust Zuul jobs filters for composer-validate and php55lint so they get
applied to the 'mwgate-*' variants.
Switch mediawiki repos to use those variants in gate-and-submit, this
way other projects would not ended up being merged in the 'mediawiki'
Drop a few "check: jsonlint" that duplicated mw-checks-test template.
Promote test_only_mediawiki_projects_in_mediawiki_gate which was only
run with 'tox -e qa'.