More helpful text for "Unable to decrypt message"

### Your use case

#### What would you like to do?
Make users stay calm and understand the situation when a message is not decrypted.

#### Why would you like to do it?
Because it's a hi-tech situation that most people don't understand, and it makes them scared. They think our product is complicated to use.

#### How would you like to achieve it?
Instead of :

Display :

In french : "En attente de ce message. Ceci pourrait prendre un moment. En savoir plus"

- the text gains a little wait time (users understand that it's expected that messages are not there yet). Same role as a spinner.
- the link gives users more info about what to do if the message is really stuck (typically, they erased cookies or logged out and in, unaware that it would lock their messages)

Note : The help link needs to be configurable. We link to our help page : https://aide.tchap.beta.gouv.fr/fr/article/messages-verrouilles-comment-les-recuperer-atnp15/

### Have you considered any alternatives?

Display a spinner to make users wait.
Display more info about progress of the sync (e.g. "36% done"), or lack of progress (message cannot be decrypted, take action to fix it).

### Additional context

_No response_