January 2019 homepage redesign

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My take on revamping the homepage to make it responsive and more obvious that pmOS is actually running on real hardware by showing featured devices. I'd like to get this out together with the new blog post (!70 (merged)). Honest feedback welcome.

Feedback wanted

• What do you think about the design?
• What do you think about the new text on the front page?
• Please test in various browsers on your PCs and phones. Does everything look good? Browse through some pages, especially the blog posts.

Commit message

Create a new design based on the Nexus 5 photos from pparent. With two featured devices, a nice quote from KDE's twitter and new about and architecture texts on the front page. Use a few numix icons (like the wiki main page). Increase the font size.

Categorize the blog posts by year in /blog. Remove TOC from all blog posts, instead add links to all headlines.

Make everything responsive by replacing the box model based layout with a new grid layout. Rewrite all CSS files from scratch, throw out (box model related) tachyons.css and main.less. The new CSS files don't need to be generated with less anymore.

Invent a new syntax to use the grid layout in the markdown blog posts by using new [#grid side#], [#grid text#], [#grid bottom#] and [#grid end#] tags. Describe it in README.md, adjust all existing posts to use grid layout. Fix a few minor mistakes in the existing posts along the way.

Update requirements.txt and document how they can be updated in README.md.

Closes #86 (closed). Fixes #87 (closed). CC @postmarketOS.