Allow Multiple Categories for File Uploads

Description:

Currently, the corpus collector app at corpus.swecha.org allows users to upload files with only one category. To enhance the flexibility and usability of the collected corpus, it is proposed that users should be able to select multiple categories for each file upload.

Tasks:

• Update the frontend to allow the selection of multiple categories.
• Modify the backend to handle and store multiple categories for each file.
• Ensure that the database schema is updated to support multiple categories per file.
• Update any relevant documentation to reflect this change.

Additional Notes:

• The user interface should clearly indicate that multiple categories can be selected.
• Provide appropriate user feedback when categories are selected or deselected.