Change icon prefix to h-icon to reduce conflicts

This should reduces the chances of style conflicts when annotator
content is injected into a host site. This pull request also ensures
all icons use the private use area of the unicode range[1].

[1]: http://en.wikipedia.org/wiki/Private_Use_Areas
Update all icon references to use the new namespace
Add an icons mixin for working with icomoon classes
Remove specific icon class from toolbar stylesheet

This allows the icon name to be changed freely without having to
update unrelated markup or stylesheets.
Remove specific icon class from markdown stylesheet
Remove self closing tags from markdown template.

Only a subset of elements are self closing[1].

[1]: http://www.w3.org/TR/2011/WD-html-markup-20110113/syntax.html#void-element
Clean up nested link styles in annotator-toolbar

Previously this generated a large amount of invalid unused css due to
nesting pseudo classes.
Namespace the pushed class in the annotator-toolbar

This will reduce the chance of styles bleeding in from the parent
document.
Rename icon-checkmark2 to icon-checkmark
Merge pull request #1559 from hypothesis/fix-icon-conflicts

Fix icon conflicts