Add Import Attachments page for Workbench

[alesan99]

Fixes #6075

TODO
Needs PR #6269 first

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add automated tests
• Add relevant issue to release milestone
• Add relevant documentation (Tester - Dev)
• Add a reverse migration if a migration is present in the PR

Testing instructions

• TODO
• Go to the workbench and click "Import Attachments".
• Select any number of files to upload.
• Set the name of the data set.
• Click Import files.
• Choose a base table
• Check that the data set contains a list of valid SpDataSetAttachment IDs (The ids can be checked with Query Builders)

[alesan99]

TODO:

• More util functions, less copy-pasted code
• Add handling for failed attachment uploads (Retry & rollback uploads if not all attachments were uploaded successfully)
• Preview files to be uploaded. A bulk upload view would be ideal, but a spreadsheet preview would be simpler.
• Add an upload progress bar like bulk attachment upload.
• Fix SpDataSetAttachments not actually being uploaded to database