Fix slow CSS selector

[nmielnik]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
New tests added?	updated
License	MIT

Description

Fixed a slow CSS selector that our linter was complaining about:

Linting dist/css/medium-editor.css...ERROR
[L209:C1]
WARNING: Unqualified attribute selectors are known to be slow. Unqualified attribute selectors are known to be slow. (unqualified-attributes) Browsers: All

To fix this, we can add a medium-editor-element class to the editor element, and update the CSS selector to use that instead of an unqualified attribute selector.

[j0k3r]

I'm ok with that 👍