[UX] CKeditor: Allow the Styles dropdown to use tag names (for easy use of the <small> tag)

Describe your issue or idea

I often want to make text in various text areas smaller. Unfortunately Backdrop's implementation of ckeditor is lacking the ability to do so :(

From stack overflow: https://stackoverflow.com/questions/24171606/ckeditor-format-tags-and-a-...

 Search for CKEDITOR.config.format_h6 in ckeditor.js and add CKEDITOR.config.format_small={element:"small"};.

open the languagefile you want (e.g. en.js) and add "tag_small":"small text". now CKEditor supports the small tag.

Steps to reproduce (if reporting a bug)

Attempt to wrap some text in the <small> tag.

Actual behavior (if reporting a bug)

There's no <small> tag.

Expected behavior (if reporting a bug)

A <small> tag.

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~PR by @quicksketch: https://github.com/backdrop/backdrop/pull/2102~ ~PR by @BWPanda: https://github.com/backdrop/backdrop/pull/2777~ PR by @quicksketch: https://github.com/backdrop/backdrop/pull/2799