Refactor Time Inputs & Hold Times

[bonedaddy]

As noted by @xiegeo in #467 (comment), there is room for refinement when it comes to how we handle hold times, and validation of them.

Day Long Increments Of Hold Time

Instead of using monthly hold times, use increments of 1 day for the pin duration.

Use time.* Types for Validation

At the moment we have a mixed bag of validating hold times, consisting of either time.* types or holdTimeInMonths int types. Instead we should have the user supply the number of days they want to pin for, and then immediately convert this to a time.Time type using time.Now().AddDate(0, 0, holdTimeInDays). We then pass this time.Time object around for manipulation, validation, etc..