UI tweaks for mobile usage (further additions)

- adjusted layout a bit more to Twitter look & feel
- switched icon theme to fontawesome (as suggested by @mvitz)
- switched to 48x48px avatar image
- set navbar to be fixed at the top
- introduced header, main and footer as requested by #118
- cleaned a bit of "divitis" in the markup
- added `<label>` element for the preferences checkbox
- added "rel=next" to the "Next" link in the footer as requested by #118

NOTE: Would still love to remove "conversation" and "in reply to" links. Any opbjections?
Adjusted this to @FND's .editorconfig