Notify that e2ee isn't supported when joining an e2ee room

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**Is your feature request related to a problem? Please describe.**
This is a hard to spot error, but hookshot can still be useful in e2ee rooms even without crypto support: once set up, post its messages unencrypted.

**Describe the solution you'd like**
Notify that e2ee isn't supported when joining an e2ee room

**Describe alternatives you've considered**
Replying to each message it can't decrypt isn't too great.

**Additional context**
Auto-creating the widget, once it supports all services, would be a great workaround. In my case it didn't do that and I needed to send the widget creation command to the room manually from an unencrypted client.

The issue names no files, tests, or entry points. Start by locating the room-join handling and the existing behavior for encrypted rooms, then define the notification shown when Hookshot joins an e2ee room; done means users are clearly informed that encryption is unsupported while unencrypted messages can still be posted.

Written by the indexing model from the issue text.