Add the section aggregate endpoints to the professor

[mikehquan19]

This pull request is for task #232. I added the section aggregate endpoints to the professor controllers. I also made the function getProfessorQuery(flag string, c *gin.Context) which gets the query of the professor depending on the flag that can be used for both professorCourse() and professorSection(). That way we can reduce redundancy in the code, though I think there's still lots of redundancy in my code.

Thank you.

[jpahm]

Awesome, thank you! Should be able to review this one much quicker than the last one :)

[jpahm]

Holding onto this until the changes discussed in #241 are made.

[mikehquan19]

I've implemented pagination on both things of the endpoints. Now the aggregate endpoints will need 2 query params former_offset & latter_offset.

For example, /course/sections?former_offset=40&latter_offset=60 would mean course is skipped to the 40th and sections is skipped to the 60th, but the number of course & sections would both be limited to 20.

A few notes:

• offset won't work in these endpoints but only the basic endpoints (/course or /professor).
• If the user doesn't specify either former_offset or latter_offset, they would be defaulted to 0. It means that /course/sections would return the first 20 sections from the first 20 course.

@jpahm , I have a question. Since I've already got the code working for #243, but since they are really messy just as we discussed, would you rather I wait until you review and approve this PR and then make a new PR, or commit them directly to this PR?

[jpahm]

Awesome, thank you! I'll review this in detail shortly. Regarding #243, feel free to either wait until I review this or move your commits for #243 to a different branch and make a PR from that.