feat: add another FooterSlot id alias

[brian-smith-tcril]

As of writing, frontend-app-learner-record doesn't use the FooterSlot component, and instead directly wraps the Footer component in a PluginSlot

The id used for that slot by frontend-app-learner-record is "footer_plugin_slot".

By adding "footer_plugin_slot" as an id alias, updating frontend-app-learner-record to use the FooterSlot component can be a non-breaking change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.06%. Comparing base (b2c55a0) to head (d86be35).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #523   +/-   ##
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  Coverage   92.06%   92.06%           
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  Files           8        8           
  Lines          63       63           
  Branches       19       19           
=======================================
  Hits           58       58           
  Misses          5        5           

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[arbrandes]

No objections. 👍🏼