Change MEDIAEMBED_THEME rendering parameter

[clarkwinkelmann]

This is more of a proof of concept because it's not really doing things the way we expect.

We should update the MEDIAEMBED_THEME rendering parameter based on the selected theme. Unfortunately I'm not sure we can change this value per-request in Flarum since the Formatter::configure extender only runs after a cache clear. There's also no way to access the actor or cookies from that extender so it would require additional code.

This PR is a working solution to always apply the default theme value as the MediaEmbed theme.

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related Flarum core extension PR's: (Omit this section if irrelevant)