Also add institution identifier to webbased invitations

[phavekes]

Add the optional institution identifier placeholder identifier in the advanced part of an invite

[MarliesRikken]

Long term preferred route is to make plugins for applications

[phavekes]

Long term preferred route is to make plugins for applications

See #538 . If this identifier is used as an placeholder, and overwritten after acceptance, this is a good solution to connect to old procedures.