ve.ui.MWTemplateDialog: Implement templatedata API fetch

Fetch
* Added basic name expansion logic since bug 48663 is not yet
  resolved in Parsoid.
* Fetch info from template data API.

Clean up (follows-up 97157a1c, de203cb8, 718db58)
* Remove redundant initialize method.
* Pass the entire target object to #getTemplateData
  because `target.wt` is *not* the template name.
* Reorganise #createPage to be more logical and rename
  to #createParamPage.
* Add todo comments for things currently missing.
* Implement error handling of promise to prevent UI from
  going stale if the promise is not resolved.
* Fix documentation in ve.ui.MWCategoryInputWidget.

Change-Id: Ie0114a81eead86d7a3b3e3a7a5b10d25c457b524
Merge "ve.ui.MWTemplateDialog: Implement templatedata API fetch"