task/ta-npe-002-36 -- factor out email subject

nparcel/templates/subject_body_html.t
nparcel/templates/subject_pe_html.t
nparcel/templates/subject_rem_html.t
nparcel/templates/subject_test_html.t
* new subject templates

nparcel/comms.py
* updated the send_email() method to build subject line from templates

nparcel/restemailer.py
* new method get_subject_line() which construct email subject line from
  a template
Only want dry tests for global unittest invocation.