Resolve home navbar issues
Resolve home navbar issues

* Turn buttons into normal links
* Use proper staticfiles lookup for image
* Use url tag for homepage
* Shorter "Start helping" text for button (for mobile UX)
* Drop "hamburger menu": Issue with BS4 prevents it being displayed
  at the very least on Android 4 on Nexus 5, making the site totally
  unusable.

volunteer-planner/volunteer_planner,alper/volunteer_planner,coders4help/volunteer_planner,coders4help/volunteer_planner,volunteer-planner/volunteer_planner,flindenberg/volunteer_planner,christophmeissner/volunteer_planner,klinger/volunteer_planner,flindenberg/volunteer_planner,pitpalme/volunteer_planner,volunteer-planner/volunteer_planner,flindenberg/volunteer_planner,pitpalme/volunteer_planner,christophmeissner/volunteer_planner,pitpalme/volunteer_planner,christophmeissner/volunteer_planner,pitpalme/volunteer_planner,alper/volunteer_planner,klinger/volunteer_planner,klinger/volunteer_planner,coders4help/volunteer_planner,alper/volunteer_planner,volunteer-planner/volunteer_planner,coders4help/volunteer_planner,klinger/volunteer_planner,christophmeissner/volunteer_planner