- added presenting list of subnets on:
  - dedicated page
  - app page
- added new ReST API group: dhcp with subnet paths and definitions
- added retrieving subnets from db with filtering by app id,
  dhcp version and arbitrary text
- switched from tabMenu to tabView for showing daemons on kea app page
  because tabMenu was messing URL of current page in the browser
- added unit tests