Issue 6749 - Made first-run page consistent with website installation flow

This MR for https://gitlab.com/eyeo/adblockplus/adblockplusui/issues/62 contains the following changes:

• Replaced existing first-run page with new content, styles and behavior
• Added web.css which contains styles taken from the website
  • Added website-defaults dependency
• Made use of newly introduced capabilities of i18n.js (see https://gitlab.com/eyeo/adblockplus/adblockplusui/commit/1e0b5f6495b8e3784d3f128f2a51bc7367d8312a) for all strings I had to touch
• Converted SVGs using SVGO (with --pretty --indent=2) but had to manually prettify the CSS code and fixed one file it broke
• Moved store badge SVGs since they're now also used outside the updates page
• Removed functions and styles that are no longer used from common.js and common.css
  • Removed duplicated style from desktop-options.css

It also contains the following (optional) small changes:

• Added support for "data-i18n-alt" attributes to i18n.js (see also https://gitlab.com/eyeo/adblockplus/adblockplusui/issues/114 for iterating on it)
• Updated string IDs to make them consistent
• Updated getDocLink() to also return a promise

Please also let me know what you think about how I ended up embedded the website code. Since I couldn't add web.adblockplus.org as a dependency, I at least wanted to make sure that we can clearly distinguish website code from our code. That's why I didn't touch any of the code I took from the homepage's stylesheet either.