Submission form workflow

[akuny]

This PR adds functionality that allows users to update GIS datasets to the application as data submissions.

• Refine DataSubmission entity
• Migrations updating the data_submissions table schema to account for the changes above
• Create FileValidator class responsible for making sure that whatever file the user chooses to upload as a data submission is valid
• Add use cases for data submissions
• Flesh out view model for data submissions
• Add routes and templates to Flask app to let users execute new use cases