Add API endpoints to frontend

[sectore]

to consume + display data.

Currently we do have following endpoints:

• categories https://github.com/aelve/guide/blob/master/src/Guide/Api/Types.hs#L48-L49
• category/{id} https://github.com/aelve/guide/blob/master/src/Guide/Api/Types.hs#L52-L53

TODOs:

• [x] Add CORS middleware
• [x] Fetch + handle data of categories endpoint
• [x] Fetch + handle data of category/{id} endpoint