Make Terminals a Separate Extension

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Problem

Terminals are not required/desired in all contexts. We discussed in last week's team meeting a path to make them optional.

Proposed Solution

• Move termainls into a separate jupyter server extension repository.
• This extension can, then, be disabled. There are certainly contexts where people don't want to allow terminals (the authorization conversations below is one instance).
• Make Jupyter Server require this as a dependency temporarily.
• In a future release, we can drop it as a dependency.

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This is now complete