Default header / footer above / below every mail

[jprusch]

As the message template used to display the mail content in MM cannot be edited w/o building the plugin, it would be great to have two additional profile settings in the [Profile:Mattermost] section:

BodyPrefix = "Markdown formatted string displayed BEFORE the mail body"
BodySuffix = "Markdown formatted string displayed AFTER the mail body"

If the mail body is not displayed, both BodyPrefix and BodySuffix should be displayed anyway.

Example:

BodyPrefix = "**[Open Mail](https://mywebmailclient.domain.com)**"
BodySuffix = "**Disclaimer: This content was forwarded to MM from XYZ and might be reformatted**"

Cause:
Our use case is to create a URL link to our web-based mail client. This is not always included in the mail body, depending who send the mail. Having this option would allow for personalized links to specific URLs. Right now, we're forwarding mails from the mail server replacing the body with a predefined text block containing the link. This is configured by user! It would be much better to handle this centrally in the mail2most configuration.

[cseeger-epages]

done v1.11.0