[Backend migration] API fetch endpoints

[tuttiq]

Part of #138 implementing a Rails backend to replace Airtable.

Implement REST endpoints to fetch all resources:

• GET /topics => returns all topics
• GET /locations => returns all locations (prefectures)
• GET /spoken_languages => returns all speaker language options
• GET /speakers => returns all speakers
  • support pagination
  • support the current search/filter capability

[ann-kilzer]

This project is now in maintenance mode, so I'm going to close some of these feature issues due to lack of resources. 🙏