mediawiki.page.ready: Use wikipage.content instead of domready

Restructure mediawiki.page.ready to add to the "wikipage.content"
hook instead of using document-ready.

Except for parts that aren't inside the wikipage content.
Portlet links are outside content entirely and should run only
once from document-ready still. Inputs with placeholders can be
both inside and outside (inside with e.g. InputBox extension,
outside in e.g. search bar of skin) so it needs to be in both.
The one in document-ready needs to exclude ones in content to
avoid applying the placeholder polyfill twice.

This also opens up the doors for extensions and gadgets to
reliably both fire and add to this hook:
- Code can fire this hook when rendering a new DOM (such as
  LivePreview, VisualEditor, ..).
- Code can add to this hook to enhance page content and have it
  properly re-run when there is a new DOM (e.g. gadgets like
  Navigation popups, Reference Tooltips, ..).

Also added release notes for 2e97025.

Bug: 30713
Bug: 33399
Bug: 51565
Change-Id: Icb0eda9edf2aeb3d612ff1d9bfea4859d33e1fbb