[Widget Mode] Updated widget mode to auto-run on page load.

[jmkulwik]

Removes the run/reset button and runs the app when the page loads.

Task: https://codedotorg.atlassian.net/browse/STAR-599

Spec: https://docs.google.com/document/d/1laYGDKIgW405YrnrWcURJDAhaKXeQPHrJ7BhyUqJMQE/edit#

Tests will be in a follow-up PR as widget mode is needed for a rapidly-approaching curriculum pilot.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (staging@0c18efc). Click here to learn what that means.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##             staging   #30138   +/-   ##
==========================================
  Coverage           ?   73.07%           
==========================================
  Files              ?     2045           
  Lines              ?   112156           
  Branches           ?     3409           
==========================================
  Hits               ?    81961           
  Misses             ?    26951           
  Partials           ?     3244

Flag	Coverage Δ
#integration	55.51% <37.5%> (?)
#storybook	56.59% <0%> (?)
#unit	58.01% <0%> (?)

Impacted Files	Coverage Δ
apps/src/StudioApp.js	63.34% <ø> (ø)
apps/src/applab/applab.js	84% <0%> (ø)
apps/src/applab/PhoneFrame.jsx	89.13% <75%> (ø)

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