Make it easier to send support email with text in subject line

Problem/Opportunity Statement

Per jetstream-cloud/project-mgt#104, Jetstream2 now requires users to specify a specific subject line when they send an email requesting support.

We currently have two configuration options for the Get Support page. Neither of them really accommodate a subject line.

  supportInfoMarkdown:
    "Please read the docs before you bother us",
  userSupportEmail: "get-support@example.org",

This raises a mild UX concern. We would give users three separate things (email address, subject line, and body) to copy and paste into the correct fields of an email message. This increases the possibility that they make a mistake or forget a field.

What would success / a fix look like?

• Provide a way for deployers to specify text that should appear in the subject line for an email to user support.
• Provide an optional mailto: link that pre-populates the user's email client with the destination address, subject line, and body.
  • This requires us to percent-escape (URL-encode) the subject and body. (Note that the body may contain arbitrary user input.)