Allow removal of agents from UI

[MatthewCroughan]

A single mis-configured agent can ruin CI.

An example would be having 10 agents, imagine you forgot to set up the binary-caches.json on one of these workers. Each derivation now has a 1 in 10 chance of failing. It would be good if we could remove these bad agents in the UI of Hercules.

[roberth]

Before this is implemented, if you can't access the agent to reconfigure or remove it, contact support@hercules-ci.com to have this done as a manual operation by us.