Avoid use of existing precompiled assets during rake assets:precompile run

Re-launch assets:precompile task using (Rake.)ruby instead of Kernel.exec so it works on Windows

To the app developer, this means configuration add in
config/initializers/* will not be executed.

Plugins developers need to special case their initializers that are
meant to be run in the assets group by adding :group => :assets.

Conflicts:

	railties/CHANGELOG
	railties/test/application/assets_test.rb

Revert "Provide a way to access to assets without using the digest, useful for static files and emails"

This reverts commit 82afaa06.

Assets pipeline: fix in manifest location and remove unused code

Fixes #2598

Otherwise, if bad data is cached in tmp/clear then the next invocation
of assets:precompile (or a regular incoming request) will pickup files from
tmp without regenerating them from source.

Otherwise, if bad data is cached in tmp/clear then the next invocation
of assets:precompile (or a regular incoming request) will pickup files from
tmp without regenerating them from source.

This makes "sprockets/railtie" explicit. This means that sprockets will
be loaded when you require "rails/all". If you are not using requiring
"rails/all", you need to manually load it with all other framework
railties.

In order to be complete, this commit also adds --skip-sprockets to
the rails generator.

See https://github.com/rails/rails/pull/1776#issuecomment-1402894,
2df2bfdb, and
7440a254.