search: refactor filedrop.js, new functionalities
search: refactor filedrop.js, new functionalities

- Refactor javascript to minimize html-side scripts
- Add sha1_git and sha256 hashing
- Completely separate text and file checksum inputs
- Add checksum validation to the text input

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui