selenium: Initial version of wdio-mediawiki package

selenium: Initial version of wdio-mediawiki package

This is new package will be reusable by other repositories for
their browser tests, without having to reference the internal
selenium/pageobjects/ directory from MediaWiki core.

In addition to not requiring direct imports, it will also avoid
problems in the future by allowing the package to be versioned
and iterated upon without forcing an atomic global upgrade
(or broken master builds), everytime we change something.

See wdio-mediawiki/README for details.

Within MediaWiki core itself, the package is used using the
'file' specifier in its package.json, so that we always test
and develop using its working copy, which makes drafting and
testing changes easier.

Also misc changes to make wdio.conf easier to understand.

Bug: T193088
Change-Id: I547a7899e7a97693a93567dd763784e637433d55