obfs4 input validation

Tor Connection should validate the obfs4 bridge line typed by the user as much as possible.

In #18579 (closed), I removed the line "Only display obfs4 bridges start with the word". As part of this issue, we should redesign the error messages.

The importance of this already got lower with persistent bridges (#5461 (closed)). It will get even lower with QR code (#18219 (closed)), Snowflake (#5494), Moat (#15331), and opening the Unsafe Browser from Tor Connection (#18339 (closed)).

We should reject this issue as soon as the situation makes it not worth it to solve this issue.

At some point, we should even remove the manual input field to avoid providing an alternative that is much less helpful than all the other ones.