Add attachments to existing row in workbench

[alesan99]

Fixes #6078

• TODO:
• Remove existing attachments from row.
• Delete attachments when a row is deleted. (Maybe do this on the backend? Simple, but could be very inefficient to read entire spreadsheet to find out if an attachment is being referenced.)
• Edit attachment order (Maybe a future PR?)
• Switch attachment in attachment viewer
• Save the dataset once an attachment has been added (Prompt user to save before uploading an attachment?).
• Cleanup (Split up big files, simplify arguments being passed into functions)

Checklist

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

Testing instructions

• Go to Workbench -> Import Attachments
• Upload multiple attachments.
• Select an existing row and add a new attachment
• Make sure the dataset was saved once the attachment was uploaded.
• Select an existing row and add multiple new attachments.
• Add an attachment to a new row

[alesan99]

• TODO:
• Remove existing attachments from row.
• Delete attachments when a row is deleted. (Maybe do this on the backend? Simple, but could be very inefficient to read entire spreadsheet to find out if an attachment is being referenced.)
• Edit attachment order (Maybe a future PR?)
• Switch attachment in attachment viewer
• Save the dataset once an attachment has been added (Prompt user to save before uploading an attachment?).
• Cleanup (Split up big files, simplify arguments being passed into functions)