TP: root tenant should not be editable in TP

[lbathina]

I'm submitting a ...

• [ ] bug report
• [ ] new feature / enhancement request
• [X] 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
• [X] Traffic Portal
• [ ] Traffic Router
• [ ] Traffic Stats
• [ ] Traffic Vault
• [ ] unknown

Current behavior:

Root tenant is editable in TP altough there is no way the root tenant could be updated.

Expected / new behavior:

It would be good to have root displayed as non editable entry

Minimal reproduction of the problem with instructions:

Anything else: