Remove redundant dir_exists call in ContentsManager.new_untitled

[jhamet93]

Problem

There is redundant ContentsManager.dir_exists in the POST API handlers for interactions with the ContentsManager. The handler calls ContentsManager.dir_exists and the ContentsManager.new_untitled calls ContentsManager.dir_exists when creating the new file as a result of this API call.

For implementations that lean on storing data in remote locations such as blob storage systems (GCS, S3, etc) or any custom implementation that involves hosting data not on disk, the duplicate invocations of these methods on POST have a performance impact. For example, at Twitter we persist internal user's data remotely so these calls add latency to the user experience of creating new files with the JupyterLab FileBrowser.

Developers do have the ability to negate this redundant call by overriding the method but this seems wasteful since it would essentially be a copy/paste of the current implementation sans the dir_exists call which most likely will result in code drift if changes are ever made to the upstream implementation.

Proposed Solution

• Eliminate the ContentsManager.dir_exists call in the ContentsManager.new_untitled implementation in the base ContentsManager classes.
• Add a ContentsManager.dir_exists call in the PUT handler to keep that behavior consistent

Alternative Solution

• ContentsManager implementations should solely be in charge of these checks rather than the API handlers to avoid this potential duplication

Additional context

I am more than willing to implement this if it seems like a desired solution.

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