JITMs: Allow JITMs to not have a dismiss button.

[kwight]

Only display the dismiss button in a JITM if is_dismissible is truthy. This will be the default setting for messages in the engine.

This will allow JITMs to be defined without a dismiss button, meaning users will continue to see the JITM until clicking on a CTA. The use-case is notices such as accepting new Terms of Service: we want the user to explicitly click the "Accept" CTA, or continue to see the JITM until new terms come into effect.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

See: p58i-8v7-p2

Testing instructions:

• On a WP.com sandbox, create a new test JITM in jitm-engine.php with a ->is_dismissible( false ) property and high priority.
• Sandbox the API, apply D38330-code, and load your JP site's valid JITM target page.
• Verify the JITM displays without a dismiss X on the right side.
• Remove the ->is_dismissible property, reload, and verify the JITM displays the dismiss button by default.

Proposed changelog entry for your changes:

• Allow hiding the dismiss button in JITMs.

[jetpackbot]

	Warnings
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[gwwar]

@kwight I'll check in with Jenny to see if we need to hide the dismiss

[jeherve]

This works well in my tests.