Add leadership flag to mission slots

[MorpheusXAUT]

Adding a leadership flag to a mission slot would allow us to add extra functionality that's limited to leadership only (e.g. a separate briefing, extra information, etc.) that would be hidden to "regular" players. Only users with an assigned leadership slot would have access to the restricted information then.

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Tasks

• Add leadership flag to mission slot
• Prepare mission details endpoint to allow for separate leadership data retrieval