Align Link Preview Toggles across platforms

We've had movement across platforms on the Link Preview toggles and need to ensure we're in alignment and they're acting as expected...

Web currently has: Global Settings to enable and disable for all rooms for unencrypted rooms and encrypted rooms.

Web currently has 2 toggles:
- on/off in unencrypted rooms
- on/off in encrypted rooms

It is not possible for a user to have only encrypted room link previews enabled therefore this should be a 'scale' not a toggle.

We want to introduce a radio component to settings where the options are:
- off
- on in unencrypted rooms
- on in all rooms

We don't show previews if it was sent by a bot and this solves the per-room toggle request that we had.

These changes will be made on web and hope to align us with mobile!

Start by reviewing the existing web Link Preview settings and comparing them with the corresponding mobile behavior. Confirm how the current unencrypted and encrypted-room toggles are represented, then define the three requested radio options and verify that bot-sent previews remain disabled. Done means web and mobile behavior are aligned.

Written by the indexing model from the issue text.