Test for importing webapp externally

[mickmister]

Summary

In order to share frontend code between calendar projects, we need to have a way to reference the source code of the shared package. This PR introduces a postinstall script that checks out the shared repo as an npm dependency, so that the common calendar logic can be imported by the gcal repo. By defining the postinstall script, we do the following whenever npm install is run:

• Clone the mattermost-plugin-google-calendar (should be mattermost-plugin-google-shared) repo as an npm dependency
• Install that project's npm dependencies

The index.tsx file on this bare-webapp branch does not define a Plugin object to provide to registerPlugin. It instead imports the one provided by the cloned module, and uses that to call registerPlugin. Note that the commit 157e796 being imported avoids calling registerPlugin in its index.tsx file

Ticket Link

[mickmister]

We would actually have this be something like mattermost-plugin-calendar-shared

[mickmister]

Also the name customModule is arbitrary

[mickmister]

Import the Plugin class from the external repo, rather than defining our own

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7de5e03) 11.22% compared to head (14e7924) 11.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
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  Coverage   11.22%   11.22%           
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  Files          16       16           
  Lines         606      606           
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  Hits           68       68           
  Misses        536      536           
  Partials        2        2           

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