Screenshot Testing - Firebase Functions

[devversion]

Talked to @tinayuangao about PR #3628.

Things need to be done after the PR is merged:

• Moving the setLocalFilenames method into the Firebase Functions
• Separating Firebase functions into different JS files (clarity)
• Setting Github Status from Firebase Functions (using firebase config)

Some additional things I would really like

• Making the Firebase "routes" more like a REST API.
  e.g upload/screenshots/${PR_NUMBER}/images

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