Drop the legacy style build steps

[mixonic]

Opened as a PR, but we should not merge this.

• 0.7.5 was never released, although there is a commit referencing it.
• I propose to release this branch as 0.8.0 (edited from 0.7.6 at @bantic's suggestion)
• Then I propose we merge this into master
• Then I propose we cut 0.8.1 with the unreleased "0.7.5" content and the contents of this page.

Drop a bunch of legacy build stuff. Stop requiring sass to be installed, just require ember-cli-sass which should permit the consumer to decide which sass implementation to use.

Drop autoprefixer because consuming apps can pick what to do on their own.

Drop some default postcss options because they make old assumptions about browser compat, and presume postcss is even used by a consumer.

[bantic]

The dropping of node-sass-import-once in v0.7.4 (https://github.com/Addepar/addepar-style-toolbox/pull/31/files) ought to have made it a bigger version change, imo. It changes how the sass is compiled for consumers that use @import, and could break client styles (as it did for our consuming app, for example).

I think we should release this branch as 0.8.0 and the follow-up merge-to-master as 0.8.1.

[bantic]

Looks good, although I think that we should start any new versions based on 0.7.4 code at 0.8.0, as I mentioned in the comment

[mixonic]

0.8.0 released with the SHA of this branch, then I merged it into master manually.