Load RTL CSS files

[Aljullu]

Fixes most issues (but not all) from #1776.

This PR:

• Installs webpack-rtl-plugin so we generate an automated RTL CSS file. It uses a fork of the plugin, as it's done in Gutenberg (Add automated RTL support WordPress/gutenberg#3844).
• Loads RTL CSS files when using an RTL locale.
• Removes legacy RTL styles from our current CSS.

Screenshots

Detailed test instructions:

• Install an RTL language in your WordPress, for example, Arabic.
• Go to Settings and change the default language to that one.
• Press Ctrl+U to view the page source and verify wc-admin CSS files contain rtl in their name.
• If you want, you can take a look at RTL issues #1776 and verify most issues are gone! 🔥

[psealock]

Nice work with this one! I just checked against issues in #1776 marked with this PR (1781) and its looking great. Thanks for handling these cases.

Just one comment about targeting the develop branch.

[psealock]

Should we target master ?

[Aljullu]

I chose that one to keep consistency with Gutenberg:
https://github.com/WordPress/gutenberg/blob/master/package.json#L123

Ideally, we would be using webpack-rtl-plugin from NPM, but we need this PR to be merged upstream before: romainberger/webpack-rtl-plugin#15.