fix: correct value for config variable `UPLOAD_FOLDER`

[sebastianliebscher]

SUMMARY

This PR fixes an incorrect default value for config variable UPLOAD_FOLDER:

• current superset/app/static/uploads/
• new superset/static/uploads/

Additionally:

• static/uploads/ is now checked into git (analog to superset/static/assets/)
• static/assets/ was initially checked into git years before, but in another commit (also years before) then .gitignored without removal from git (I think unintentionally)

The config variables IMG_UPLOAD_FOLDER and IMG_UPLOAD_URL were never used (found nothing using full text search) and are therefore removed.

TESTING INSTRUCTIONS

1. docker build . -t test
2. run integrations (+unit) tests based on that docker image
3. look for correct directories

ADDITIONAL INFORMATION

• [ ] Has associated issue:
• [ ] Required feature flags:
• [ ] Changes UI
• [ ] Includes DB Migration (follow approval process in SIP-59)
  • [ ] Migration is atomic, supports rollback & is backwards-compatible
  • [ ] Confirm DB migration upgrade and downgrade tested
  • [ ] Runtime estimates and downtime expectations provided
• [ ] Introduces new feature or API
• [ ] Removes existing feature or API