Adjust credits for special courses

**The feature**
Add the ability to adjust/override credit hours for special courses.

**Detailed explanation**
Special courses include research and special problems. I had this issue when registering for CS 4699 which is presented as a 1 hour credit course when it can be up worth up to 12 hours. This feature is also documented in #41 but I wanted to make a separate issue for it to address soon.

**Mockups**
It appears that courses with a schedule type of "Directed Study" in Oscar are the ones whose credit hours can vary. Courses can be filtered for this field and users can be given the option to adjust their credits according to their own schedule using a slider, similar to how prerequisites are shown.

Start by locating the course registration flow, course filtering by schedule type, and the existing prerequisite slider or similar control. Read issue #41 for related requirements and confirm how credit hours are represented. Done should include a way to adjust credits for Directed Study courses, including the CS 4699 use case, without changing fixed-credit courses.

Written by the indexing model from the issue text.