Allow enabled in remote development agent configuration to be set to false

MR: feat: Allow enabled in remote development confi... (!137119 - merged)

Description

As a user, I want to disable remote development feature of the agent.

Context

• &11516 (comment 1567563355)
• #402131 (comment 1567522234)

Agent's admin view should show a banner that the remote development feature is disabled and thus no actions can be performed from the admin view. We've updated the issue description to reflect this. The documentation should be updated to reflect this.

Acceptance Criteria

• Agent configuration should allow setting remote_development.enabled to false.
• This should stop the agent from communicating with Rails.
• This should stop creating new workspaces associated with the agent. Note: New issue created to stop showing workspaces where the enabled flag is false in the dropdown when creating a new workspace.
• Documentation is updated
  • Add a new section Disabling the agent to the documentation to explain the consequences of toggling the remote_development.enabled flag.
  • Update the agent configuration to point to this section so that agent administrators are well aware of the behaviour, since the consequence of not being aware means that any non-terminated workspaces will run and continue to incur infra costs indefinitely. Note: New issue created for doc updates to keep the MR to Issue 1:1.