Add filtering by status to signups

[hujoseph99]

Ticket link

Closes #126

Implementation description

• To add filtering by signups to our getShiftSignupsForUser endpoint, I added an optional argument for specifying the status of the signup.
  • getShiftSignupsForUser(id) -> getShiftSignupsForUser(id, status)

Steps to test

1. You can populate the db into a testable state by running the commands found in this PR
2. Create some signups using mutations. For example: mutation { createShiftSignups( shifts: [{ shiftId: 3, userId: 1, numVolunteers: 3, note: "test" }] ) { shiftId userId numVolunteers note } }
3. Test various scenarios to see if the filtering works as expected.

What should reviewers focus on?

• Test to see if the filtering works as behaved.

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[LenaNguyen]