Fixes #114 - animates closing of trail details

- Changes how trail details animates by utilizing the `top` property
and `calc()` notation to reduce the number of variables in the
setTrailViewOffset() method.
- Performs broad CSS by removing unnecessary properties.
- Performs HTML cleanup by removing unneeded nesting and assigning IDs
to elements that are retrieved by JS.
- Migrates some HTML elements around the trail details to semantic
elements so the nesting of elements is easier to see in the source HTML.
- Adds methods to controller for toggling trail details, and
consolidates code around toggling the trail details. Also, creates
variables for DOM elements that were looked up repeatedly.

Of note, the trail details page is no longer hidden with ng-show; it’s
simply moved out of view. This is to accommodate animating it out of
view when the view changes to the map view. It’s at a 3000 z-index, so
anything that should appear on top of it should be greater than
z-index:3000.
Removes margin doubling

Margin has been added to the trail details container, so individual
elements in the container don’t need to set their margin in order to
align. This commit removes the individual element margins.
Adjusts trailhead-title-field area

Adjusts positioning of trailhead-title-field area to accommodate
different title field lengths.
Merge pull request #126 from codeforamerica/114-animate-trail-close

Fixes #114 - animates closing of trail details