Improve Django Admin Subject Input Form

[alfonsrv]

Currently Django admin's subject input form is very limited when it comes to having multiple OIDs such as OUs and DCs. While limiting functionality the currently used widget also adds a lot of complexity to the overall codebase. (discussed in #75)

Some solutions from the top of my head would be:

• Improve the form to support Formsets with e.g. Django Dynamic Formset – Pro: best solution -- Con: tedious to implement
• Replace the currently used widget with a single CharField – Pro: easy to implement & most flexibility; and arguably most usable too, as it's mainly power users that use CAs in the first place -- Con: UX suffers
• Give users the ability via a Checkbox to simply adopt the CSRs subject, so no form-input is required for more complex Subject names – Pro: nice-to-have feature, easy to implement -- Con: additional form complexity for the user and code-base