ve.ui.MWFormatTool: Update heading size CSS for new OOUI version

* Use .oo-ui-tool-title-text selector rather than .oo-ui-tool-title
  to avoid styling accelerator keys.
* Divide all sizes by 0.8, as we no longer need to match the font-size
  rule on .oo-ui-tool-title.
* Add CSS for heading1 (which differs between Vector and core VE)
  and heading2 (because it looks silly if it's missing).

Change-Id: I12226f0a674037f904149cc10893f5c154767bfc
Merge "ve.ui.MWFormatTool: Update heading size CSS for new OOUI version"