Internships are offered for complex periods, for example: "Every Q1 and Q2" "Q1 every year" "Q1, Q3 this year, Q2, Q4 next year" We need to offer support for at least some of these. At minimum: * Select quarters where the internship is offered, assume this repeats every year (note: might not be quarters, but rather other periods, set these on course edition?) * Set a date as of which the internship will no longer be offered * Set a date as of which the internships will be offered (adds support for Always in Q1 and Q3, but not the upcoming Q1) This feature consists of multiple parts: * [ ] Add registration of offering information (internally) * [ ] Add way to enter this conveniently * [ ] Display offering information to staff and students * [ ] Apply offering information to only make correct internships available to students (/ internally copy internships, further discussion necessary for determining best approach)