ref: migrate createPresignedPost endpoint for images to TypeScript

[karrui]

Problem

This PR migrates the createPresignedPost endpoint flow to Typescript (barring router layer).
Adds tests for all the new migrated functions.

Solution

Features:

• add ExternalError class for errors arising externally (like S3)
• add AdminFormService#createPresignedPostForImages function
• add AdminFormController#handleCreatePresignedPostForImages function

Improvements:

• replace controller handler function used for POST /:formId([a-fA-F0-9]{24})/adminform/images route with migrated TypeScript function
• Remove all traces of old Javascript handler function

[karrui]

Turning into draft, realize I can kill two birds with one stop by doing presigned URL for logos too without much increase in code lol

EDIT: extracted out common code, will still do logos in a separate PR

[mantariksh]

suggested minor improvements!