Upgrade CodeMirror

[microbit-matt-hillsdon]

There are breaking changes that will affect us in various trivial ways:
https://discuss.codemirror.net/t/release-0-20-0/4302

I think we should hold off on this for a few weeks, not least because it might affect the Oxford student team building on the editor, but we should do it in time to fix any issues surfaced by V3 live release.

[microbit-matt-hillsdon]

We should be good to look at this from the 9th May.

[microbit-matt-hillsdon]

This should fix #753 due to codemirror/view@6db2d95 assuming that makes it into a patch release before too long.

[microbit-matt-hillsdon]

There's also a subsequent 1.0 upgrade now so two sets of notes to look at in the forum.