User or room specific "Never send encrypted messages to unverified sessions"

Suggestion

Currently there is only an option to "Never send encrypted messages to unverified sessions from this session" which only applies to that Element Web instance.

I would like to mark users such as myself using cross-signing as "encrypt only to verified sessions" and the very few people I have happened to meet during the pandemic instead of having to remember doing this for every room with those people.

As an example to only encrypting to own verified sessions, I tend to have a room only for myself and different accounts on all protocols so I can quickly throw notes and similar there when I just need to transfer it to another device quickly and easily. I also have hanging unverified sessions as some Matrix clients are yet to implement emoji/qr verification or clearly showing their details for manual verification.

Related issues:

• https://github.com/vector-im/element-meta/issues/1114 - about the opposite, when the existing option is checked, the other end sees unclear error messages.
• https://github.com/matrix-org/matrix-doc/issues/3354 - collection of other encryption issues, some of which have been fixed.

No files, tests, or entry points are named. Start by reading this proposal and the linked Element Meta and Matrix specification issues, then determine the required protocol behavior and specification changes. Done means the user- or room-specific verification policy is defined consistently with existing encryption and cross-signing rules.