Enable URL previews in encrypted rooms

### Your use case

#### What would you like to do?
Add the option to enable url previews to encrypted rooms, on a room-by-room basis.

#### Why would you like to do it?
This would bring element-ios to feature parity with element-web. For security reasons the option is off by default, and must be enabled per room, as the home server is asked to fetch preview data. Once enabled on web, the preference is synced on all web clients.

#### How would you like to achieve it?
Add a toggle to Room Details -> Settings that reflects the URL preview setting, and if enabled fetches the URL preview from the homeserver like the web client.

### Have you considered any alternatives?

If bots were permitted in encrypted rooms, you might be able to achieve functionality that way, but sadly they are not. I'm not sure there is an alternative, since it's a request to bring feature parity with element-web.

No files or tests are named. Start at the Room Details → Settings flow and compare the web client's per-room URL-preview behavior; done means an off-by-default toggle for encrypted rooms that enables homeserver preview fetching and reflects the synced room preference.

Written by the indexing model from the issue text.