Fix private message text for webconf invitations, refs #1020

Wasn't sending the message written by the user when sending the invitation
via private message. Via email was sending it correctly.
Fix wrong message in space invitation emails, refs #984

Was sending the message in the wrong language in some cases, and in some cases
using more than one language in the same email.
This was in the email sent to the admins of a space confirming that a user
accepted or rejected an invitation to join the space.