Cannot add a server capability to a server in TP

[mitchell852]

I'm submitting a ...

• bug report
• new feature / enhancement request
• improvement request (usability, performance, tech debt, etc.)
• other

Traffic Control components affected ...

• CDN in a Box
• Documentation
• Grove
• Traffic Control Client
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault
• unknown

Current behavior:

Cannot add a capability to a server in TP.

Expected / new behavior:

You should be able to add a capability to a server. This is a regression caused by #4747 . More specifically it's due to the removal of this method - https://github.com/apache/trafficcontrol/pull/4747/files#diff-58e3f2204371520d8da9abcd1e5339c2L33

Minimal reproduction of the problem with instructions:

• Navigate to a server and select Manage Capabilities
• Attempt to add a capability to a server
• Observe the 'serverId' cannot be blank error.