DataImport payload cleanup

Remove a few unused payload settings from the previous Git import and the elements import.

See Markdown file: https://notes.vpn/M-Lwo0X8S_60j6_gmZjpHg?view

• Remove repo_id from the backend
• Remove sha from the backend
• Remove element_name from tasks
• Rename elt_type to element_type for the Elements imports
• Remove corpus_id from the backend
• Add a wiki page for the payload docs : https://wiki.vpn/en/arkindex/dev-doc/dataimport-payload

Follow-ups:

• Renaming folder_id to parent_id for explicitness
• Payload validation per import type using a custom JSONField
• Consider dropping DataImport altogether, or at least drop the payloads?