Show students when self-reservation window will open

[chrishtr]

Suppose a student has an upcoming exam, but the self-reservation window hasn't opened yet. If the student visits the PrairieTest homepage expecting to see a "Make a reservation" option, they'll see this instead:

While the message is correct, it doesn't help students or their instructors understand why there's nothing reservable, especially in a situation where an instructor tells students that something should be available to reserve but the self-reservation window wasn't configured appropriately.

The home page should be updated to show students upcoming exams and when their self-reservation window will open.

[nwalters512]

Some notes:

• This will increase PrairieTest's susceptibility to the "Ticketmaster problem", where we end up with a sudden stampede of reservation attempts when self-reservation windows open. We recently had a minor production impact event due to lock contention because some institutions started announcing when reservation windows open to their students, which triggered such a stampede. Showing the dates to all students (even at institutions that historically didn't tell students about the windows) would only make this worse. We should fix that lock contention issue first. For those with access: https://github.com/PrairieLearnInc/PrairieTest/issues/2722.
  • This will generally change student dynamics compared to the
• Jim at UIUC noted that this would help them as well. Despite Illinois courses generally not announcing the start of self-reserve windows, students who aren't familiar with the reservation system end up emailing either the CBTF or their instructors thinking that something is wrong because they can't immediately reserve whenever the happen to sign into PT. Showing a start date would help with this.

Jim shared that a course that he took would just have a date marker in the course calendar - "Register for your exam on Tuesday April 14" or such. We could refuse to show a specific time and just say "Available for registration starting April 14" (before the day) or "Available for registration later today" (day of). But this would be suboptimal for institutions that do have a culture of announcing specific times, and could cause extra stress for students even at institutions that don't.