WebGroup: join group web message improvements
WebGroup: join group web message improvements

* To invite new members to join an existing group, the only possibility
  is to write a web message.  However, no predefined text was available
  before.  This is now implemented with a default message containing
  a link to be followed in order to join the group. (closes: #8243)

inveniosoftware/invenio-accounts,inveniosoftware/invenio-accounts,tiborsimko/invenio-accounts,tiborsimko/invenio-accounts,tiborsimko/invenio-accounts,inspirehep/invenio-accounts,inveniosoftware/invenio-accounts,inspirehep/invenio-accounts,inspirehep/invenio-accounts