Consolidate the requests required to start up the client app

[jacobwod]

This is not nice:

I have mainly two concerns with the loading sequence above:

1. Three request go out from the same API - i.e. they could be consolidated server-side and sent back as one request
2. Response to config/layers includes the entire layers directory for all Hajk maps on this server - not only those used by this specific map config. This leads to unnecessarily large file size.

This can be fixed pretty easily by modifing the backend and adjusting load sequence in client.

My intent is to fix this for the NodeJS-based backend. Feel free to fix the .NET-backend if you're depending on it.

[jacobwod]

Quite an improvement: customeTheme.json is unchanged, but the remaining requests (31,5 kB + 1,65 MB) are now down to 196 kB.

[jacobwod]

This is now merged into master. For activation, add the following line to client's appConfig.json:

"experimentalNewApi": true,