The "modules/context.go" is too large to maintain.
This PR splits it to separate files, eg: context_request.go,
context_response.go, context_serve.go
This PR will help:
1. The future refactoring for Gitea's web context (eg: simplify the context)
2. Introduce proper "range request" support
3. Introduce context function
This PR only moves code, doesn't change any logic.
