[FEATURE] Django/DMOJ-like permissions

[jdabtieu]

Currently, CTFOJ only supports three permission levels: user, admin, and superadmin. Superadmin is hard-coded as the user with ID 1, so really there are only two permission levels that can be set. We also should have problems manager (can add/edit/delete problems but without full admin interface), perms for individual contests/problems, etc.

• Shift permissions to a new table of [user_id, permission_id]
• Remove the current admin flag
• Add permission enums (SUPER_ADMIN, ADMIN, PROBLEM_MANAGER)
• Add a problem_authors table mapping [problem_id, contest_id (optional), user_id] to set problem authors, which also implicitly gives authors perms to their own problems and displays authors on the problem page
• Add a contest_admins table mapping [contest_id, user_id] to give contest admins effective admin perms for their own contests
• Add a content manager role with permissions to manage pages, contests, problems, but not users
• Permission inheritance?

[jdabtieu]

Content manager role to be added in 4.2.0, other tasks are still scheduled for 4.3.0