Adding non-urgent message alert to secure messaging form

[jmarchi]

Description of Change

Added an informational alert to the secure message form that the messaging feature is meant for non urgent messages.

Screenshots/Video

Testing

Added a unit test for the display of the alert.

• Tested on iOS
• Tested on Android

Reviewer Validations

PR Checklist

Reviewer: Confirm the items below as you review

• PR is connected to issue(s)
• Tests are included to cover this change (when possible)
• No magic strings (All string unions follow the Union -> Constant type pattern)
• No secrets or API keys are checked in
• All imports are absolute (no relative imports)
• New functions and Redux work have proper TSDoc annotations

For QA

Run a build for this branch

[dumathane]

Left some notes.

I'm also interested if anyone asked if we should be applying a similar message to reply's or editing drafts as they are technically different screens. I'd assume the start new message is the important one it should apply to, but want to hit parity if we can with the web teams.

Also did anyone ask if we should be removing the old place we housed this notice? I might start a thread today with those questions.