Allow for configuring url preview Accept-Language headers on a per-user, per-room and per-group basis

This issue has been migrated from [#7277](https://github.com/matrix-org/synapse/issues/7277).

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As of #7265, a server admin can specify the preferred language that URL previews should appear in. However, you may have a homeserver made up of people who speak multiple languages, who would prefer URL previews to appear in their own language.

Ideally you could set a default URL preview language in your client's settings, in room settings, in a group's settings, and the server would figure out which language best to request for you from the remote server.

Will require spec work and making sure this works with other media repository implementations.

Previous discussion: https://github.com/matrix-org/synapse/issues/3037