[header] Hide header on scroll if app is rendered in mobile device. Closes #452.
[header] Made two instances of Headroom to support #content and #browser containers as scrollers. Related to #452.
[header] - Use component/dom instead of slifszyc/query  #452
Merge pull request #521 from DemocracyOS/452-header-autohide

Autohide header. Closes #452.