REL: 1.2.3: CSV special case corrections, extra data in notifications, documentation

• Corrections to handling of extra data in mailgen template preview
• Corrections for special cases in CSV data
• Extended Documentation

Backend

• convert example data to Event to handle extra data in database insert:
  for the mailgen preview, convert the example data to an IntelMQ event
  and before submitting it to the database, to a dict with extra as a
  string.
  Previously, extra-data provided by the user could not be inserted,
  raising an error

Frontend

• frontend: detect if csv data header can be used as field name and use it:
  if the csv column name (table header) contains a legitimate IntelMQ
  field name, use this one and skip the further sanitation steps
• frontend: handle CSV data with no body, but only with a header:
  if the CSV data contains just one line and the header option is active,
  the data is body/content-less
  handle these special cases by aborting the parse and disabling the
  spinner

Documentation

• docs: document format spec (mailgen) influence
• docs: change URLs to rendered sphinx docs in README