plugin email invitations: re-use existing invitation for a given email: if an invitation is done on an email which has already received an invitation, it is re-used and no new XMPP account or Libervia profile is created (the name is modified though). This is currently done in an inefficient way due to `LazyPersistentBinaryDict` limitations (all invitations are loaded and checked to find the email). A more efficient check should be easier to implement with storage changes in 0.9 .

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)