Add Controlled Digital Lending Values to Avalon Config

[joncameron]

Description

Adding a new ability to configure in Avalon's settings file for controlled digital lending, as well as an option value to set the application-wide default for lending periods and value for maximum number of items a patron may borrow at one time.

Done Looks Like

• [x] A key exists in settings to enable/disableControlled Digital Lending
• [x] Lending period (if this key does not appear, assign "14 days" as the default)
• [x] Maximum checkouts per user (if this key does not appear, assign "25" as the default)

Something along the lines of:

controlled_digital_lending:
  enable: true
  default_lending_period: 14 days
  max_checkouts_per_user: 25

[cjcolvar]

ISO8601 Duration (e.g. "P14D" or "P2W") ActiveSupport::Duration.parse("P14D") == 14.days DateTime.current + ActiveSupport::Duration.parse("P14D")