FIX IE 11 rendering issues in Page Editor UI

[Cheddam]

This PR, alongside silverstripe/silverstripe-admin#931, aims to drastically improve the usability of the Page Editor UI in Internet Explorer 11, particularly at mid-sized resolutions. In particular, the changes here:

• Set a specific base width on the two main elements holding the editor and preview pane, so that Internet Explorer doesn't collapse them.
• Removes the fill-width class from the editor header, which was causing its tabs to collapse onto and overlap the second-level tabs.

There is still an outstanding IE issue where the Page Editor UI stops adapting to smaller viewports at around 850px, but with this PR users will have a far more usable experience. I'm also seeing some sizing issues when the Elemental Editor is present, but the UI is usable and fixes itself when the viewing mode is changed.

Resolves #2432.
Resolves #2245.

[brynwhyman]

Wondering if this might help resolve this issue too: #2345

[robbieaverill]

Who do we appreciate

[NightJar]

Tested (with companion PR silverstripe/silverstripe-admin#931), can recreate neither #2432 nor #2245 anymore, well done @Cheddam you hero ❤️ 🎉

[NightJar]

Nice, and with minimal changes, that was unexpected!

[robbieaverill]

This fixes the CMS content editing in IE11, nice. I've found it breaks the split screen view of page history though (taken on MacOS Chrome, and shows up in IE11 when you load the history page directly rather than with PJAX)

The history screen is still pretty broken in IE11 when you load it with PJAX:

Also when you load the history tab with PJAX there's this weird loop issue where you can click on the split mode toggle and send it into a crazy loop: https://screencast-o-matic.com/watch/cqjDcVtn73