Fetch Skylight native assets when compiling

[JonRCahill]

When previously compiling if the native Skylight assets were not present then only the bootstrap files for the Skylight app would be compiled with a message prompting to download the native assets using mix skylight.fetch. This had the following issues:

• Running mix skylight.fetch would download to the local project directory rather than the Skylight dependency directory meaning the native assets would never be available
• Elixir considers the Skylight app as compiled, so any other time you try to compile (mix compile/mix phoenix.server/mix test etc) Skylight is never recompiled with the native assets so is never available in your app. The only workaround for this is to run mix deps.compile or something similar to force a compile

This pull request then changes compiling the Skylight app so it downloads, extracts and compiles the native assets to fix all the above issues. Additionally, if any of these critical steps cannot be completed then the elixir compile explicitly fails so the recompile issues don't arise.

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[wagenet]

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