Release Track Endpoint Summary Updates

[clemiller]

Requests to add the following data to release track endpoints to support frontend views:

/GET /api/release-tracks

• Counts for number of candidates, staged, and members for each release track

GET /api/release-tracks/:id

For each item in the candidates, staged, and members lists:

• ATT&CK ID
• Object name
• Object description (if any)
• Last modified by User information (at minimum, the FE needs the user display name, or if that's not available, the username)