Refactored navigation bar for better mobile devices support

[luccioman]

• Moved the search input to the nav bar.
• Removed hacky absolute positionnings.
• Cleaned custom css on elements.

This works now better especially on android devices (on many of them, the search button and toggle button were hidden before this).
Still to improve :

• when a result snippet is very long, the search button is scrolled on the right
• main landing page : YaCy logo is maybe too much close to the left border
• page is quite long to load with low bandwidth

Tests :

• BrowserStack mobile devices
• a real Orange Dive 30 (android 5.1)