mw.ViewPageTarget: Show save errors in save dialog instead of alert

To trigger a save rejection from the api, set:
 $wgSpamRegex = '/spam/i';
and making an edit adding the word "spam" to a page.

Class changes:
* Rename message system in the save dialog from "warning" to
  "message" as it will now contain both warnings and errors.
  (css class, class property, method names, ..)

Localisation:
* Remove ugly hardcoded and wikitext-requiring "'''Warning:'''"
  prefix from the warning message, instead have a message for the
  word "Warning" and re-use this in #showMessage for each message
  of type "warning" (bolding applied in code instead of in i18n).
* Rename visualeditor-savedialog-dirtywarning to
  visualeditor-savedialog-warning-dirty and remove from
  VisualEditorMessagesModule.php as it no longer requires pre-
  processing from the server.

Clean up:
* Re-alphabetise the order of some messages.
* Clean up duplication and redundant logic in mw.Target#onSaveError
  and mw.ViewPageTarget#onSaveError.

Bug: 50350
Change-Id: I3daf631fb0d62ba88e05aa50c77c9940d61395a0
Merge "mw.ViewPageTarget: Show save errors in save dialog instead of alert"