Add pool tags to db schema

This adds a new column to the database to store tags for pools, just
like it's done for prefixes and VRFs!

Part of #566.
Add pool tags to backend

Backend now supports storing tags for pools, just how it's done for
prefixes and VRFs.

Part of #566.
Add pool tags to pynipap

Pynipap now supports tags for pools!

Part of #566.
Add pool tags for CLI

CLI support for tags on VRFs, similar to how it works for prefixes and
VRFs.

Part of #566.
Add pool tags to web UI

Just like prefixes and VRFs, we not got tags on pools!

Part of #566.
Fix test case for pool tags

Fixes #566.
Merge pull request #582 from plajjan/566-add-pool-tags

Add pool tags