Add support for email notifications

[atc0005]

SUMMARY

This commit adds support for sending email notifications using the
same behavioral feature set as Microsoft Teams notifications:

• configurable retries and retry delay
• configurable rate limit
• a separate notification per event
  • e.g., request received, username disabled, sessions terminated

The following settings are configurable:

• destination email address(es)
• sender address
• SMTP server IP/FQDN
• SMTP server port
• client identity
  • hostname provided to the SMTP server
  • as noted elsewhere, some SMTP servers will perform forward/reverse
    DNS lookups to confirm that this provided value matches what DNS
    has recorded for the sending IP Address

Some known weaknesses/lack of support:

• hard-coded email template

  • as of this commit, the template using Textile formatting is
    enabled
  • future efforts are intended to move hard-coded templates out of
    source code constants to on-disk files for easier customization
    by the sysadmin(s) supporting this application

• unauthenticated connections to SMTP server

  • the initial use-case is a local relay which restricts connections
    based on source IP Address
  • future efforts are intended to provide support for authentication

Behavior worth noting:

• automatic client identity value generated from available DNS
  information if not specified by the sysadmin
  • the Showmax/go-fqdn package is used to retrieve the FQDN of the
    sending system, with a fallback to
• email address verification applied using the provided W3C regex
  • using a guide from golangcode.com

CHANGES

The following items note (non-exhaustive) changes since the last
point release. Many of these changes will see further refactoring
as part of GH-22.

• Duplicate client/alert request headers so that in-place formatting
  modifications performed on values in the slice do not spill over to
  the values used for email notifications (where a different
  formatting is used)

• Replace placeholder "getter" methods with full-featured equivalents
  to existing MS Teams methods

• Add new "getter" methods to cover new functionality

• Add flags, environment variables to cover new functionality

• Rename existing generically named functions used for Teams
  notification handling to explicitly mention relation to Teams
  notifications

  • this was needed to make room for email-specific notification
    functionality

• Rework some existing Teams-specific functions to allow use with
  Teams and email notifications (DRY)

• Add email-specific functions, methods, types to mirror existing
  Microsoft Teams notifications support

• Replace placeholder email notification template with a "default",
  mostly text-only friendly format and a Textile formatted template to
  be used with older Redmine systems (prior to Markdown support being
  introduced)

• MessageTrailer func updated to take a template string for
  generation of a format-specific branding string

  • e.g., Textile-formatted repo link or Markdown-formatted repo link

• Update config Stringer interface

  • it is growing quite large at this point, might be time to
    rethink how it is handled

• TOML configuration file

  • extended with new settings
  • rename some settings in an effort to better communicate intent

• Documentation

  • Cover new flags, environment variables and config file settings
  • Misc fixes for existing rate limit, number of retries and retry
    delay
  • Refresh existing setup/deploy steps to briefly cover email
    configuration

REFERENCES

• fixes Feature: Add support for email notifications when disabling user #3

• https://golangcode.com/validate-an-email-address/

  • https://www.w3.org/TR/2016/REC-html51-20161101/sec-forms.html

• https://github.com/Showmax/go-fqdn