Add caption for chart panels

[IshavSohal]

Related Item(s)

ramp4-pcar4/storylines-editor#393

Changes

• Added captions for chart panels

Testing

Steps:

1. Open the demo page
2. Navigate to the dynamic panel with title This Slide Is Dynamic!
3. Open the charts panel
4. Observe the caption under each chart of the slideshow
5. Navigate to the Highcharts Demo slide
6. Observe the caption under each chart of the slideshow

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This change is 

[yileifeng]

Please exclude the package-lock.json file from PR

Reviewed 4 of 4 files at r1, all commit messages.
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[IshavSohal]

Donethanks

Reviewable status: 3 of 4 files reviewed, all discussions resolved (waiting on @yileifeng)

[yileifeng]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @IshavSohal)

[RyanCoulsonCA]

With the current implementation, the caption won't be visible if there's a single chart on a slide with a background image:

The solution we use for the image panel should work here as well. You should also add the text-sm class to the caption so it's consistent with the other captions.

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @IshavSohal)

[IshavSohal]

Donethanks

Reviewable status: 2 of 4 files reviewed, all discussions resolved (waiting on @yileifeng)

[RyanCoulsonCA]

Reviewed 1 of 4 files at r1, 1 of 1 files at r2, 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @IshavSohal)