Better way of URL-Preview

#### Is your feature request related to a problem? Please describe.
I'm always frustrated when my users complain about URL-Previews being not available in encrypted chats on Android (and iOS).

Probably related to anything there: https://github.com/vector-im/element-android/issues?q=is%3Aissue+is%3Aopen+url-preview

#### Describe the solution you'd like.
I would like to have an Administrator setting per room to enable a per-user setting for this room. So the Administrators get to decide if the users of a room can individually enable/disable URL-Previews for their client. (defaulting to: disabled each)
This should be consistent for all Element-branded Matrix clients.

#### Describe alternatives you've considered.
Alternative: a "show preview" button added by the client to each message containing an URL (for each url in that message or at least the first 3 URLs). Possibly enabled by an Adminstrator setting per room.

#### Additional context
URL-Preview is kind of inconsistent between Element-branded clients. Also users might not want to forgo client-side URL-Previews.