Landing page send message

[lnielsen]

• Send message feature

[lnielsen]

From inveniosoftware/invenio-app-rdm#347:

As someone finding a record of interest, I want to contact the person responsible for the record by clicking on a "Contact" button, so that I can discuss the record with them.

person responsible for the record: owner, proxy, contact email ?
contact button: leads to a text area to write up the message
stale emails: if the contact email is stale, it would be nice to send an email to alert the site's administrators (that email should always be valid ;) ). (don't know yet if possible...)