[UX] Design a single modal dialog for E2EE setup/import

[aguinetqb]

Today we have two commands to setup e2ee:

• /init generate a new key & publish it
• /import imports a previously backuped key

We have errors messages depending on the current situation, with --force flags to enforce the command if we know what we are doing.

We should do only one command & one modal dialog that adapts regarding the situation:

• if no public key exist for the user:
  • if no key is in the browser: generates & show the backup
  • if a key is in the browser: it probably means something wrong happened potentially during key generation. Propose key generation again.
• if a public key exists for the user:
  • if a key is in the browser, verify that it matches. If not, propose to import the according private key or override the public key. If it matches, everything is fine (do nothing)
  • if no key is in the browser, propose to import a key. Add a button to force key generation with the appropriate warning (that is the inability to read the user's old all messages for the user itself and others)

Once this dialog exists, we could also add a menu entry in the main MM menu to launch it, making this far more user friendly!