Add a <kbd> button to the editor toolbar

[a2sheppy]

This adds an option to the toolbar, adjacent to the <code> button,
which applies the <kbd> element to content. The <pre> button is
also moved to the right end of the toolbar button group it's in,
to place it closer to the syntax highlighting buttons.

This resolves these two bugs:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1209767
• https://bugzilla.mozilla.org/show_bug.cgi?id=1251506

[a2sheppy]

I have no ability to set reviewers on this so consider this comment a request that one of @schalkneethling, @jwhitlock,or @Elchi3 give this a review. :)

[schalkneethling]

After rebuilding the CKEditor source I can confirm that the editor toolbar is updated as expected:

@a2sheppy Just want to confirm that all relevant files are in the PR. The docs at https://kuma.readthedocs.io/en/latest/ckeditor.html#committing-changes state the following:

When updating CKEditor, adding a plugin, or changing the configuration, CKEditor should be rebuilt, and the results added to a new pull request.

Are there additional build files that need to be added to this PR, or is it not applicable? Thanks!

[a2sheppy]

Updated with the built CKEditor.

[schalkneethling]

@a2sheppy Was this intentionally added?

[a2sheppy]

No... the instructions said that a git add on the plugins folder would add only the files that need to be, so I just did that and went with it. Didn't notice this going in there.

[schalkneethling]

Seems like a lot more files was added causing a whole list of conflicts. I wonder if all of these are required? https://kuma.readthedocs.io/en/latest/ckeditor.html#committing-changes

@jwhitlock Thoughts?

[a2sheppy]

So, there's been another change to CKEditor since I initially submitted this PR, apparently. I will attempt to deal with it.

[a2sheppy]

I have sadly exceeded my git skills at this point and am not sure how to proceed to fix this. I'm sorry about that. :(

[jwhitlock]

I don't think it would be a good use of your time to try to repair this PR.

Here's the quick version:

1. Start over with a branch from the current master on the Kuma repo
2. Cherry-pick a941e9a onto that branch
3. Re-build
4. Submit a new PR

[a2sheppy]

Will do. That's what I was thinking.