Prepping to reduce merge conflicts between this PR and Submitty#12931 by matching the format of the code shared between them.

### Why is this Change Important & Necessary?
Currently while auto-open is enabled, opening one or more new files in
the gradeable interface will flush the list of auto-opened files that
were opened in previous viewed submissions. This reduces the usefulness
of auto-open for gradeables where submission files can be named
differently.

### What is the New Behavior?
Previous files designated as auto-opened will persist when additional
files are opened while auto-open is enabled. Any files that you closed
will be removed from the list of auto-opened files. The list of
auto-opened files will be flushed when you disable and re-enable
auto-open.

### What steps should a reviewer take to reproduce or test the bug or
new feature?

1. on main, create a new gradeable where students can submit files
2. have one student submit a non-image file, and another student submit
another non-image file with a different name
3. open the grading interface to the first student, enable auto open,
and open the student's submitted file
4. navigate to the other student, and open their submitted file
5. go back to the first student

the first student's submitted file WILL NOT be auto-opened

7. now go to the PR branch and repeat steps 3-5

the first student's submitted file WILL be auto-opened

Additional testing:
- Test that auto-open works with nested folder structures (by submitting
a .zip of a folder with files and following a similar switching
procedure to the one described above)
- Test that image file behavior is unchanged (image files are not
auto-openable)
- Test that disabling auto-open stops any files from being auto-opened,
and re-enabling auto-open does not result in any files being auto-opened
by default (because clicking the auto-open checkbox flushes the
auto-open list)

### Automated Testing & Documentation
The current auto-open Cypress test has been updated to test this new
behavior

### Other Information
This PR changes the name of the "files" cookie to "open_files" for more
clarity on its purpose; please let me know if you have wording
suggestions.
This PR also removes mentions of the "opened_mark" and "scroll_pixel"
cookies, which are never generated and may be leftovers from features
that are no longer supported or desired. I have left behind the
unfinished "testcases" cookie logic, as it seems to be mostly completed
and just needs to be set properly. Please let me know what you think of
keeping/removing these cookies and if another PR to implement them might
be worth it.

---------

Co-authored-by: Barb Cutler <[email protected]>

* main: (78 commits)
  [Bugfix:TAGrading] Auto-open single file (#12625)
  [Feature:InstructorUI] Add sortable columns to manage students (#12957)
  [Dependency] Bump php-ds/php-ds from 1.7.0 to 2.0.1 in /site (#12827)
  [Refactor:TAGrading] Events Infrastructure & StatusBanner (#12941)
  [Dependency] Bump sqlalchemy from 2.0.48 to 2.0.51 in /.setup/pip (#12884)
  [Bugfix:Developer] trust hashicorp for vagrant install (#12968)
  [Feature:TAGrading] Backend for submission clustering (#12886)
  [Bugfix:InstructorUI] Missing Subsection in the Edit User form (#12955)
  [Bugfix:Submission] Warning Banners Accessibility Fix (#12956)
  [Bugfix:System] Preserve signup fields (#12939)
  [Bugfix:InstructorUI] Fix Csv Download of Subsections (#12954)
  [Bugfix:InstructorUI] Fix Manage Students Toggle Columns (#12953)
  [Feature:TAGrading] Persistent Auto-Open (#12931)
  [Bugfix:Submission] Dot File Upload (#12924)
  [Refactor:System] Declutter workers (#12815)
  [Bugfix:Developer] save/restore autograding_containers.json (#12952)
  [Bugfix:InstructorUI] Create Course Validation (#12949)
  [Testing:Notifications] Fix grade_inquiries.spec.js (#12950)
  [UI/UX:System] Add fullscreen button vue component (#12936)
  [Bugfix:TAGrading] Student Name Resize (#12946)
  ...

# Conflicts:
#	.pylintrc