gql: add field "assignedSchedules" to type "User"

[Forfold]

Description:
Adds the field assignedSchedules to type User so that each schedule's shifts can be fetched for a given user. This will be especially important for adding a calendar to a user's profile, so they know the context of each shift they are on-call for.

Which issue(s) this PR fixes:
This PR is precedent to #3229 and is a part of #3164

Screenshots:

Other changes:
Also adds the argument user on query shifts to use for filtering:

type Schedule {
  // ...
  shifts(start: ISOTimestamp!, end: ISOTimestamp!, userIDs: [ID!]): [OnCallShift!]!
  // ...
}

[allending313]

lgtm!

[mastercactapus]

I propose we expand the existing shifts field on Schedule by adding a userIDs argument that will filter the returned shifts to the provided list.

Secondly, an assignedSchedules field on User that will return the list of schedules the user is associated with (i.e., assigned directly, on a future add_override /w end_time in the future, or participant of a rotation assigned to the schedule)

type Schedule {
  // ...
  shifts(start: ISOTimestamp!, end: ISOTimestamp!, userIDs: [ID!]): [OnCallShift!]!
  // ...
}

type User {
  // ...
  assignedSchedules: [Schedule!]
  // ...
} 

[Forfold]

When attempting to find any temporary schedules, the application gets stuck in an infinite timeout. No idea why. It happens on this line:

err := s.grp.Get(ctx, timeKey("userIDs", time.Minute), groupcache.AllocatingByteSliceSink(&data))

[Forfold]

Blocked by #3286

[ethan-haynes]

LGTM 👍

[ethan-haynes]

Small comment, but should we avoid "magic numbers" like the 50 used here and above in end.After(start.AddDate(0, 0, 50?

[Forfold]

Looks like it's an issue with most of our calls to validate.ManyUUID, so I'm going to document that for its own issue, if possible

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