Fix employees can see postings outside of branch

Ticket link

Closes #647

Filter by user branches in postings resolver if user is volunteer or employee
- If employee also only return scheduled or past postings
- If volunteer also only return unscheduled or scheduled postings
- Removed old way of doing this for volunteers where we pass an optional userId parameter
Updated VolunteerPostingsAvailabilities page to redirect to not found page on posting error
Updated SchedulePostingsPage to redirect to not found page on posting error
Updated branches query only returns branches that user is in

Log in as employee
Check that you can only see postings of branches you are in
Log in as volunteer, and check that you can also only see postings of branches you are in

[x] My PR name is descriptive and in imperative tense
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[x] I have run the appropriate linter(s)
[x] 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