Better manage re-connecting to a non-existing server

If the server stored in the storage is not accessible anymore when launching the app again, we end up with a blank screen for 5 minutes (default HTTP timeout of the JS client).

To do:

• set the HTTP timeout to 30 seconds, that's probably enough.

When trying to re-connect to a server:

• display waiter
• display a message "Connecting to SERVER_ NAME"
• after 10 seconds, add another message "Seems that it's difficult to connect"
• add an action (ghost button) RETRY => go to Login with the stored server URL prefilled.