Add description field to Server Capabilities API

[rawlinp]

This Feature Request affects these Traffic Control components:

• Traffic Ops
• Traffic Portal

Description

A Server Capability today is just a name and nothing else. In a lot of cases, a name is descriptive enough, but in some cases, naming a capability is more difficult and doesn't fully describe the capability itself and how it needs to be assigned to certain servers. With a description field, we can keep names more succinct while being able to fully describe the capability itself and how it should be assigned to certain servers, including why it was even necessary to create the capability in the first place.