mailutils: Flask-Email initial port
mailutils: Flask-Email initial port

* Adds new configuration variable CFG_MAIL_BACKEND.  (closes #1531)

* Adds html and plaintext email templates.

* Adds unit tests for sending emails.

* Adds `mailtutils_backend_adminonly` for sending emails only to
  CFG_SITE_ADMIN_EMAIL.

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base