This commit adds `ActionView.deprecator` and replaces all usages of
`ActiveSupport::Deprecation.warn` in `actionview/lib` with
`ActionView.deprecator`.  This commit also replaces a call to Ruby's
`Module#deprecate_constant` with Rails' `DeprecatedConstantProxy`, so
that its deprecation behavior can be configured using
`ActionView.deprecator`.

Additionally, this commit adds `ActionView.deprecator` to
`Rails.application.deprecators` so that it can be configured via
settings such as `config.active_support.report_deprecations`.

This commit also removes a few defunct `assert_deprecated` calls that
were not failing because they were nested in `assert_raises`, and the
raised error prevented checking the deprecation.  (One was mistakenly
kept in d52d7739 when converting
`test_render_file_with_errors` to `test_render_template_with_errors`;
the other two were added in dd9991ba but
not removed when the deprecation was completed in
85ecf6e4.)