Properly determine a submitted quiz

[merkushin]

Resolves #7276

Proposed Changes

• Add a method to the quiz progress to determine if the quiz has been submitted.
• Fix legacy check of the status.
• Fix pagination not showing in an edge case (when a lesson was completed, but then a quiz was added, we visit the quiz on the frontend —it is not submitted yet— and see all the questions on one page).

Testing Instructions

1. Publish a lesson.
2. Complete the lesson as a student.
3. Add a quiz with a few questions and pagination to that lesson and update it.
4. As a student, go to the quiz.
5. You should see proper placement of buttons in the footer (quiz template) and paginated output of questions (for example, you see only one question per page if you set it that way, so you don't see all questions on one page).

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7278 (852b4a2) into trunk (8e68ffa) will increase coverage by 0.27%.
Report is 7 commits behind head on trunk.
The diff coverage is 77.94%.

@@             Coverage Diff              @@
##              trunk    #7278      +/-   ##
============================================
+ Coverage     50.59%   50.86%   +0.27%     
- Complexity    10975    11022      +47     
============================================
  Files           608      610       +2     
  Lines         46286    46552     +266     
  Branches        402      402              
============================================
+ Hits          23417    23678     +261     
- Misses        22542    22547       +5     
  Partials        327      327              

Files	Coverage Δ
includes/class-sensei-lesson.php	36.20% <100.00%> (ø)
includes/class-sensei-quiz.php	69.48% <100.00%> (ø)
...es/course-theme/class-sensei-course-theme-quiz.php	44.00% <100.00%> (+44.00%)	⬆️
...urse-theme/class-sensei-course-theme-templates.php	14.04% <100.00%> (+3.40%)	⬆️
...z-progress/models/class-quiz-progress-abstract.php	86.79% <100.00%> (+9.01%)	⬆️
...-progress/models/class-quiz-progress-interface.php	50.00% <ø> (ø)
...i/class-sensei-rest-api-question-helpers-trait.php	92.78% <100.00%> (+0.80%)	⬆️
includes/class-sensei-cli.php	0.00% <0.00%> (ø)
...des/cli/class-sensei-validate-progress-command.php	0.00% <0.00%> (ø)
.../class-sensei-validate-quiz-submission-command.php	0.00% <0.00%> (ø)
... and 1 more

... and 4 files with indirect coverage changes

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[m1r0]

Looks good to me.