`ADD` function for condition Date comparisons in permission conditions

[robere2]

Having the ability to offset a datetime (more specifically the $now variable) in permission conditions would allow us to restrict users permissions for only a certain interval of time, e.g., only being able to view Productions less than 7 days before they're scheduled to start.

Possible syntax could look something like:

{
  "startTime": {
    "lt": "ADD($now, 604800)"
  }
}

604,800 is the number of seconds in 7 days.

This should also work with normal datetime strings, but there's not much purpose to doing so:

{
  "startTime": {
    "lt": "ADD('2024-04-01T18:20:31+0000', 604800)"
  }
}

[robere2]

Having this ready for Fall 2024 hockey season would be ideal.

[robere2]

First referenced in #11