Move admin UI assets to the admin service directory

[gunnaraasen]

It's been bothering me that the admin interface assets have been living in the root of the project. This moves them into the admin service directory, which feels like a more intuitive location for these files.

Also, the asset embedding step has been converted to use go generate instead of calling statik directly.

@rossmcdonald @toddboom

[rossmcdonald]

+1, I agree that services/ is a much better location.

[toddboom]

👍