resourceloader: Replace ResourceLoaderDebug config use with context

resourceloader: Replace ResourceLoaderDebug config use with context

Reduce our reliance on static state and configuration, and
propagate more state in explicit ways, through context, and
request parameters.

OutputPage creates ResourceLoaderContext and ResourceLoaderClientHtml
based on the configuration (via ResourceLoader::inDebugMode).

Everything within those classes should not need to check it
again.

* ResourceLoaderClientHtml:
  Already doesn't check MW config, but it's test was still
  mocking it. Removed now, and confirmed that it passes both
  with true and false. The individual test cases set
  debug=true/false as needed already.

  It's sets were previously relying on the accidental behaviour
  that within a unit test, we don't serialise over HTTP, which
  meant that a pure PHP boolean would survive. With the new
  raw `=== 'true'` check, this no longer works. Set it as a
  string explicitly instead, which is the only thing we support
  outside unit tests as well.

* ResourceLoaderContext:
  Remove fallback to MW config when 'debug' is unset.
  This is never unset in practice given that all load.php
  urls have it set by OutputPage based on ResourceLoader::inDebugMode.

  This change means that manually constructed ad-hoc load.php
  urls that are missing 'debug=' parameter, will now always be
  read as debug=false. This was the default already, but could
  previously be changed through wgResourceLoaderDebug.

When changing wgResourceLoaderDebug, everything will still have
debug=true as before. The only change is when constructing load.php
urls manually and explicitly not set it.

Bug: T32956
Change-Id: Ie3424be46e2b8311968f3068ca08ba6a1139224a