Adds simple mattermost notifier

[ruupert]

🚀 Changes proposed by this PR

Adds a simple Mattermost webhook notifier. Changes:

• Plugs into sendQueue() keeping email notifications first to be processed.
• Configuration follows the same pattern as for Config.Email with Config.Mattermost.
• Does not introduce any new external mod dependencies

🔗 Related bug reports/feature requests

• relates to Add support for additional notification methods #3

🧰 Type of change

• New feature (non-breaking change which adds functionality).

📝 Notes to reviewer

Not asking to merge this but rather just a review in case you have any concerns or issue with the way this change fits in.

Might have to move email and mattermost handling into own functions and in sendQueue() have it like:

var err = error
if conf.Email.Enabled {
    err = errsendEmailNotification(text)
    // if err, log it
}
if conf.Mattermost.Enabled {
    err = sendMattermostNotification(text)
    // if err, log it
}

so that each can return on errs

🤝 Requirements

• ✍ I have read and agree to this projects Code of Conduct.
• ✍ I have read and agree to this projects Contribution Guidelines.
• ✍ I have read and agree to the Developer Certificate of Origin.
• 🔎 I have performed a self-review of my own changes.
• 🎨 My changes follow the style guidelines of this project.
• 💬 My changes as properly commented, primarily for hard-to-understand areas.