mediawiki.jqueryMsg: Phase out redundant data module and minor clean up

mediawiki.jqueryMsg: Phase out redundant data module and minor clean up

Follows-up 4a3e50a54.

* Merge mediawiki.jqueryMsg.data and mediawiki.jqueryMsg modules.

  There's no need for this to be a separate module. The data is not for public consumption,
  it's provided to jqueryMsg only.

* Remove unused default-default values for 'allowedHtmlElements'.

* Remove conditionals around data providing at initial run-time. Instead, expose
  private method can call that. This way, we don't have two code paths claim
  ownership over the namespace. And it makes the module easier to test and re-use
  by not requiring the data to exist at first run time.

* Fix getDefinitionSummary() implementation to append data instead of setting
  arbitary keys in parent data. ResourceLoader documentation of getDefinitionSummary()
  has been updated to reflect this practice.

Change-Id: I40006d39514a997dce4930756a3dac84a0c9bb83