Merge WooCommerce Dark Mode changes

[AmandaRiu]

This PR merges in the changes from the WooCommerce dark mode project contained in these two PRs:

• Dark Mode: Base login screens
• Dark Mode v2: Site address help

As well as this older unrelated PR that must've been missed for merging back to this repo:

• Gradle version upgraded to 3.5.3
• Fixed bug when the forgot password url in username password screen was not valid
• Added logic to fetch the correct SiteModel from the local db for the incoming url

To Test

I've opened a PR in WPAndroid to ensure these changes doe not conflict with that app. Please visit that PR to test.

** Only one developer required to review and approve this PR

[renanferrari]

I've tested this changes with WPAndroid and everything seems to be working fine. :shipit: