[Backend] Change reports to data files

[carltonsmith]

Description:
Based on user research, we have changed reports section on the frontend to "Data Files". Having the name differ between the frontend and backend makes the code more difficult to understand and maintain. Also, it is likely that down the road the application will have to support reports on the data in the system, and this name would be better used at that point.

Acceptance Criteria:

• All references in code to reports are changed to data_files
• Database tables and fields are updated so any reference to report or reports are changed to data_file and data_files respectively.
• Testing Checklist has been run and all tests pass
• README is updated, if necessary

Tasks:

• Task 1
• Task 2
• Task 3
• Run Testing Checklist and confirm all tests pass

[riatzukiza]

This work would be complicated to do with all of the outstanding work on 398. We would have to edit several different branches, and put the effort into making sure each branch is caught up with their dependencies. We should wait until all currently outstanding prs for 398 are merged.

[jtwillis92]

We should do this as a fast follow after ATO, prior to uploading any actual data files into the system - that way we don't have to run migrations to move the data files to the new model / django app.

[jtwillis92]

@amilash wanted to put this one on your radar too. If we get to a point where we are building reports for data analysts to download this would get very confusing without the rename. While it is renamed in the visible portions of the UI - there is a lot of frontend and backend code that refers to it by this name as well. The earlier we do this refactor the easier it will be.

[jtwillis92]

Note, unless #1061 gets merged before this work is started you will likely want to branch from there to avoid missing the backend changes from there.