Allow Banners to render basic markdown such as links and emphasis.

[TomTirapani]

• Added reactMarkdown component and dependency.

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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
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[amcclain]

Thank you for jumping on this - has been on my list for a while to discuss built-in markdown support, and this was a good prompt to do so.

• Should we consider publishing a markdown component outside of the kit directory? I am not sure - I can't think of anything really to add or normalize on the API, but at the same time we typically publish a non-kit cmp for components we expect developers to be using regularly.
• Not for the banner usage, but thinking of other in-app usages like comments, we could consider adding https://github.com/remarkjs/remark-breaks as a preconfigured plugin. Github makes this modification to standard MD in a few places and I always find it intuitive:

By way of example
this is on another line
(but wouldn't be in standard markdown, I don't think)

• Changelog entry should include it in a Libraries section + a callout in the New Features section to recommend that apps cut over to the Hoist export + remove from their package.json to take advantage of our managed upgrade cycle (compatibility testing, etc) and to avoid potentially including multiple versions of the library should we have non-overlapping version specs.
• I'm all for adding this as a built-in, but we should check to how much this increases the bundle size of e.g. Toolbox. It looks like a fair amount of dependencies, and with the usage within the built-in banner, we know it will always be included in any/every app bundle.

[amcclain]

Another thought - at some point it would be great to have a user-friendly MarkdownInput component with something like the formatting toolbar in this Github commenting box.

Codemirror has a markdown mode and we could certainly configure that, but then you are in monospace/non-wrapping "code editing" mode and that's not really the experience I think we would want for user (or even admin) facing MD editing.

Not something we need to do for this PR, but it could be a companion comp we could consider when we go to use MD more widely for use cases like in-app comments (something I'd like to offer in one of our current apps).

[amcclain]

We need to wrap the "full" display in the new markdown comp, for a multi-line banner prompt. First line shows up in banner, rest of the message (which now might include more advanced MD formatting) shown in a pop-up.

[TomTirapani]

Thanks for taking a look Anselm - I've updated the PR. Including the remark-breaks plugin was justification enough to wrap this in its own Markdown component. I've updated the CHANGELOG entry, and ensured the "Read More" popup is also rendered as markdown.

[TomTirapani]

According toolbox/yarn startAndAnalyze with the inlineHoist flag set to true, this PR increases the bundle size 23.81 MB > 24.39 MB

[lbwexler]

This looks great to me --- we might want to add some text in the banner admin discussing what is going to work, and what won't. Happy to merge this now

[amcclain]

Thanks for getting those additional changes in 👍