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