(P2) Allow admins to configure in site config whether clients can use chat/autocomplete/commands as features.

Allow site admins to put in place restrictions on which Cody features may be used client-side in VS Code, etc.

i.e. the ability to globally turn on/off the following for all users (or better, for groups of users, if easily implementable):

• Chat
• Autocomplete
• Commands
• TODO: ask Ryan Phillips if any other functionality is missing here.

This will require e.g. adding a new site config option, and propagating that to VS Code clients to have them decide which features users can use based on the Sourcegraph instance.

It may be a good starter task for someone looking to dive in to both codebases.