Add category configuring, and default accept unofficial to true for active elements

[lainets]

Description

What?

Add easier category configuring, and accept unofficial submissions by default to active elements exercises without max submissions.

Why?

To close #165.

How?

Active elements with max submissions are put in a separate category "active elements with max submissions" that doesn't accept unofficial submisions by default.

Closes #162 Closes #164 Closes #165

Testing

What type of test did you run?

• [ ] Accessibility test using the WAVE extension.
• [ ] Django unit tests.
• [ ] Selenium tests.
• [ ] Other test. (Add a description below)
• [X] Manual testing.

Tried changing the category options in conf.py, and checked that the changes happen in A+. Also checked that "active elements" category accepts unofficial submission by default and that it can be overridden in conf.py.

[lainets]

Of note is whether the solution to the max submissions/accept unofficial submissions problem is satisfactory. Does changing the category of active elements exercises with max submissions cause issues to some courses?