* FIX: Mark invites flash messages as HTML safe.
This change should be safe as all user inputs included in the errors are sanitized before sending it back to the client.
Context: https://meta.discourse.org/t/html-tags-are-explicit-after-latest-update/214220
* If somebody adds a new error message that includes user input and doesn't sanitize it, using html-safe suddenly becomes unsafe again. As an extra layer of protection, we make the client sanitize the error message received from the backend.
* Escape user input instead of sanitizing
5ee31cbf7d