Add basic integration with opensea

[unjapones]

Connects #312.

Includes:

• Add CollectiblesMetadataGateway to /src/services: actions/thunks should interact with this service.
• Add folder /src/services/collectibles_metadata_sources: here we will add the different sources of metadata information (for now: opensea, mocked).
  • Add CollectibleMetadataSource, interface that these "sources" must implement.
• Add COLLECTIBLE_SOURCE constant (updated .env.example) to set the source that the gateway will use.

Does not include (yet):

• Dynamic/async setup of source based on networkId.
• Tests.