Update middleware validator to handle only min value, add min value validations in various forms

[ankurp-bst]

1. Updated the readMinMax function to handle empty max value so that only min value can be provided (for example: a field that requires positive number, hence min=0 and no max value). For this field the validation will be described as:

type Example struct {
    PositiveField int `valid:"0,"`
}

Here only min value (0) will be validated, and if negative value is entered, it will show an error:

Positive Field: should be at least 0

2. Several forms on the control panel didn't have proper validations on the backend, this PR adds those validations.

[jo3-l]

Consider letting readMinMax return math.MaxFloat64 if there is no explicit maximum given, Then there will be no need to specially handle the case where there is no maximum.
Never mind; this doesn't simplify the code much at all. Disregard.

[jo3-l]

It's possible to simplify this thusly: return min, max, valid.values[1] == "". Whether this is a good idea is debatable, but figured I'd throw it out there.