Use native Netlify functions instead of Gatsby functions

[cesarvarela]

closes #3017

should be merged after #3005

Requires the following environment variables to be added to Netlify UI:

REALM_API_APP_ID=
REALM_API_GROUP_ID=
REALM_API_PRIVATE_KEY=
REALM_API_PUBLIC_KEY=
REALM_GRAPHQL_API_KEY=
REALM_APP_ID=
API_MONGODB_CONNECTION_STRING=
ROLLBAR_POST_SERVER_ITEM_ACCESS_TOKEN=

Can be seen working here: https://pr-158--cesarvarela-staging.netlify.app/apps/submitted/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.43%. Comparing base (2174d92) to head (76da015).
Report is 1 commits behind head on staging.

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #3018      +/-   ##
===========================================
- Coverage    83.59%   79.43%   -4.17%     
===========================================
  Files          165      160       -5     
  Lines        13267    13143     -124     
  Branches      1528     1486      -42     
===========================================
- Hits         11091    10440     -651     
- Misses        1960     2410     +450     
- Partials       216      293      +77     

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