Customizable browser spell-checking

[#5452](https://github.com/matrix-org/matrix-react-sdk/pull/5452) adds support for multiple spell-checker languages to the desktop app. We should investigate the usage of the `spellcheck` and `lang` attributes for configurable browser spellchecking in whatever shape or form. We should also keep in mind that most browsers allow you to change the spell-checker language already (usually using a context-menu).

UX possibilities:
+ Per room (it might make sense to have an MSC to define the language of the room instead of each user setting this separately)
+ Override browser context-menu