ve.ui.FormatTool: 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 (and round to 5%), as we no longer need to
  match the font-size rule on .oo-ui-tool-title.

Change-Id: I4dd3c55017e7edc25c7af30b5f4c9e103a475016
Merge "ve.ui.FormatTool: Update heading size CSS for new OOUI version"