Use JavaScript to show/hide fields when switching assignment_type.

[somiaj]

When selecting the assignment_type on the ProblemSetDetails page, use JavaScript to show/hide set fields for the appropriate assignment type. This way one can change a set's assignment type and edit the specific assignment type fields without having to save the changes first to see the fields for that type.

[somiaj]

I just realized this isn't complete or might be not possible, because this only messes with set fields, and switching also changes problem fields. Making a draft for now as I think about what to do with the problem fields.

[somiaj]

This now shows/hides the problem fields based on set type as well, but the javascript would be a bit too complicated to update the set numbering to change to/from JITAR sets, so in that case a warning is shown to let the user they need to "Save Changes" to update the problems when changing to/from a JITAR set.

[dlglin]

This is probably beyond the scope of this PR, but how about adding the ability to choose an assignment type when creating a set?

[somiaj]

Since I cannot think of a good way to deal with to/from jitar sets and updating problems to the jitar naming/nesting buttons, this is now ready for review again. It does update problem fields, and just warns users when they need to "Save Changes" to move to/from jitar sets to fully update the problems to/from the jitar naming/nesting ability.

[somiaj]

@dlglin that could be a possibility, but there are multiple places a set can be created. Adding that option to the "Create" action tab in the "Sets Manager" would be doable. Though should this also be added when creating a set from the "Library Browser". Unsure if there is any other way a set can be created (outside of importing which already contains the type).