Run snapshot tests based on storybook

[wlach]

This provides some additional assurance that PRs don't break aspects
of the UI.

Pull Request checklist

• [N/A] Documentation: If this feature has or requires documentation, the relevant docs have been updated.
• [N/A] Changelog: This PR updates the changelog with any user-visible changes.
• Tests: This PR includes thorough tests or an explanation of why it does not

[codecov]

Codecov Report

Merging #2506 into master will increase coverage by 1.06%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2506      +/-   ##
=========================================
+ Coverage   69.74%   70.8%   +1.06%     
=========================================
  Files         223     236      +13     
  Lines        5530    5769     +239     
  Branches      894     928      +34     
=========================================
+ Hits         3857    4085     +228     
- Misses       1654    1665      +11     
  Partials       19      19

Impacted Files	Coverage Δ
.storybook/config.js	100% <ø> (ø)
src/reps/__stories__/reps.stories.js	25% <100%> (ø)
src/editor/components/modals/modal-container.js	100% <0%> (ø)
...__stories__/drop-target/components/FileManager.jsx	57.5% <0%> (ø)
...rc/reps/serialization/get-top-level-rep-summary.js	100% <0%> (ø)
src/reps/__stories__/table-rep.stories.js	100% <0%> (ø)
..._stories__/drop-target/components/ImageGallery.jsx	42.5% <0%> (ø)
...sources/__stories__/manage-file-sources.stories.js	71.42% <0%> (ø)
... and 14 more

[wlach]

Some of the reps test don't pass because they output non-deterministic stack information. Should be fixable in a followup, in the meantime this would have caught the recent react table brokenness.

[hamilton]

lgtm