gotour: update CodeMirror editor to 5.25.2

Update CodeMirror editor from version 3.16 to the latest current
version, which is 5.25.2.

The goal is to reduce maintenance burden in the future, and
make it easier to enable features like bracket matching, etc.

The update was done by overwriting existing files with those
from the latest CodeMirror release. No new files were added.
I tested gotour locally and the new editor appears to work ok.
There are no errors in console, typing works, and I noticed
at least one issue resolved in the new version. Typing
CodeMirror.version in browser console now prints "5.25.2".

Commit 768e12f applied manual changes
to mode/go/go.js, but these have been merged upstream via
codemirror/codemirror5@047afd2,
codemirror/codemirror5@524f54a, and
codemirror/codemirror5@1ec2263.
As a result, I was able to use mode/go/go.js from upstream
without cherry-picking those changes.

Helps #146.
Helps golang/go#18723.

Change-Id: I94bd687609b80de7a4ed89cf6a300a878ac9bfe6
Reviewed-on: https://go-review.googlesource.com/41207
Reviewed-by: Chris Broadfoot <cbro@golang.org>