[Feature] Right to left text authoring

[marc-hughes]

This PR adds a menu-option in slate editors, and a button on the markdown editor that will enable right to left text formatting & authoring for languages such as Arabic.

When enabled, the editor will operate in right to left mode.

If you're typing English text, it acts more like a right-aligned text with some weird rules for punctuation. Typing Latin characters is not a good test. When typing a right-to-left language, the characters will be inserted to the left of the text, as one would expect. I've downloaded some other right-to-left enabled editors, and this seems to be the correct behavior.

Delivery mode should match what you see in authoring.

Caveat: I do not know any right to left languages, so I'm solely testing this by using a foreign-language setting on my keyboard and typing gibberish, then comparing that to the same gibberish in an external editor with RTL support.

The editor will remember your last right/left selected option and attempt to create new elements using that format. This will be useful for someone creating a full course in an RTL language. It may be confusing/annoying if someone ever accidently picks the option, or if they only need RTL in a few sections.