Add Disable Personal Access Tokens setting to Admin Settings UI

What does this MR do and why?

Related to:

• Admins unable to enable personal access tokens ... (#399233 - closed)
• Allow administrators to enable or disable Perso... (#436991 - closed)

Currently personal access tokens can only be disabled instance-wide via the API. The first issue described what is perceived as a bug. Once an admin uses a PAT to disable PATs, they can no longer authenticate via the API to re-enable this setting. This leaves admins with few options, except the Rails or database console. For some customers, this is not possible. I believe using an OAuth token to authenticate to the API would work, but it requires the additional steps of creating an application, retrieving a token, and then calling the API.

The second issue is a feature proposal that adds this setting to the Admin area settings UI. This seems like the easiest route to providing a more convenient method for admins to re-enable PATs.

The setting appears under Admin area > Settings > General and only appears for instances with an appropriate Premium/Ultimate license.

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