Add VRF tags to db schema

This adds a news column to the database schema to store tags for VRFs,
much like we have tags for prefixes.

DB version has been bumped to version 5!

Part of #484.
Add VRF tags to backend

This adds support for tags for VRFs in the backend. Smart search has
been modified to support using hash (#) to search for VRFs with a tag.

Part of #484.
Add VRF tags in pynipap

Pynipap now supports tags for VRFs!

Part of #484.
Add VRF tags to CLI

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

Part of #484.
Add VRF tags to web UI

This is the final piece of the puzzle for adding tags to VRFs. Web UI
now sports VRF tags similar to how it works for prefixes.

Fixes #484.