Client awareness + ConfigSet/ConfigCheck requests

[renatomassaro]

Closes #320.

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Now Helix can support custom behaviour depending on the client platform (Web/Mobile) and the game version (1/2).

That's when we finally persist the Setup information from DLaydOS :ok_man:

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• [x] Client awareness
• [x] Client state
• [x] [Server.]Config.Set
• [x] [Server.]Config.Check
• [x] Client.Setup
• [x] Web1.Setup

Incidental

• Overall improvements on ServerService tests, API
• Core.Validator
• Moved requests from <service>/websocket/channel/<service>/requests to <service>/websocket/requests
• Request.Proxy, which dispatches the request to another request, based on select_backend/2.
• Add hostname to the Server model
• Add SetHostnameRequest

This change is 

[sourcelevel-bot[bot]]

Ebert has finished reviewing this Pull Request and has found:

• 4 possible new issues (including those that may have been commented here).
• 2 fixed issues! 🎉

You can see more details about this review at https://ebertapp.io/github/HackerExperience/Helix/pulls/331.

[renatomassaro]

Reviewed 57 of 68 files at r1, 18 of 21 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

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