Messenger: add Save Draft and Preview features

[SKuipers]

This PR makes some major usability changes the the New Message functionality in Messenger, adding save draft, test email, and preview features. In the back end, scripts have been refactored, the New Message and Edit Message pages now share the same form, and the giant "messenger_postProcess.php" file is no more. There are parts of the giant send process that I have intentionally not refactored, with the aim to not introduce too many possible bugs in this already large change set.

Description

• Changes the Submit button on New Message to two different options: Save Draft and Preview & Send.
• Adds a Prefight option to send a test email to preview the message contents.
• Drafts can be edited from Manage Messages and saved until ready to send.
• Displays an error message if a user clicks Preview & Send but hasn't selected any targets.
• Adds a Preview & Send page with a list of recipients which can be manually toggled before sending.
• Updates the Send Report page to enable re-sending even when Read Receipts are not enabled.
• Adds descriptive help text to the top of the Send Report page.
• Highlights recipients with email delivery issues on the Send Report.
• Added an SMS preview and automatically remove the signature for SMS sending.

Motivation and Context Messenger can be powerful yet daunting for users. These features aim to make it easier to use and less intimidating by giving more previews and checks before sending a message.

How Has This Been Tested? Locally.

Screenshots

[rossdotparker]

This is a massive chunk of work, and some much needed refactoring in what has become a rather unwieldy script. I've reviewed it, and can't see anything amiss. I'm going to merge now, and will then test locally.