Add enablePipeline as a flag for enabling pipeline feature

[lizable]

Is your feature request related to a problem? Please describe. For now, pipeline feature is hidden although it's applied to main branch. Let's provide a flag for pipeline feature on configuration so that the user can enable/disable the feature.

Describe the solution you'd like

• [ ] Apply AON(All-or-nothing) login when pipeline feature is enabled* (Logout when login attempt of either Backend.AI WebUI or of Backend.AI Pipeline fails)
• [ ] Hide/Show pipeline menus when pipeline feature is disabled/enabled

Describe alternatives you've considered If it is not adequate to use configuration value from WebUI, then adding configuration key/value in webserver will be an alternative.

Additional context