[FR] Add easier management UI for flags

[matmair]

Please verify that this feature request has NOT been suggested before.

• I checked and didn't find a similar feature request

Problem statement

An administrator has a wide array of conditions that can be used to select feature flag state. This, however, requires them to read the docs and jump into django admin interface.

Suggested solution

Make use of built-in mechanisms for flag management (in addition to the more technical methods):

• Add a custom condition that uses InvenTreeUserSettings to control flag state
• Add pane for users to control experimental features
• Give Admins control of which flags are shown/promoted to users
• Add flag overview to admin center

Describe alternatives you've considered

I currently have a custom plugin that pulls flag state from PostHog and control them there.

Examples of other systems

No response

Do you want to develop this?

• I want to develop this.

[matmair]

Would benefit from #2284 for promotion

[matmair]

Follow up from the docs added in #9333