Feature: Custom list properties

*This is the precursor to inventory / equipment management.*

**This is based on the client login feature and should be merge _after_ feature/client-login**

This patch allows administrators to configure custom properties for custom lists. Then, each item in the custom list can have define values for the custom properties. The values are then usable in templates.

### Features
  * Custom properties for custom lists
  * Manage values for properties for each list item
  * Use values in canned responses and email templates

### Outstanding
  * [ ] Ticket filters can use custom properties of custom lists
  * [ ] Add staff management pages to view, search, and manage items in the custom list items