Improve usability of sheet tabs

Remove the drop shadow from the highlight states

Previously the active tab would be red, which could be confused with our
red anchor text and make the other tab look disabled. The new design
makes all text gray but adds an "active" border below the button.

Simplified search

Update from error styles based on feedback

Based on feedback from Dan that the plain red text is hard to
distinguish from hyper-links.

Tweak the HTML and CSS of the new search input.

Besides the line height hack for FF kill the rest of the px related
calculation and do everything in nice, pretty ems.

The trick used here was designed to honor the font setting in a user's
browser by assuming a typical default of 16px and computing the base
font size in ems relative to that.

The problem is that even if everything is sized in ems there are
sometimes aggressive user agent styles that complicate things by
restoring the system font. It turns out it fixes the various things
I've noticed in my FF this way.

This may fix the text issue displayed in FF with custom display
settings.

https://github.com/hypothesis/h/pull/1326#issuecomment-50655944

 * Remove the background color.
 * Borders and icon darken when the input is in focus.
 * Adds the placeholder text back.

Added a readable class name to the search icon .icon-search and
regenerated the icomoon css file.

This is now a standalone module with no dependency on the Visual
Search library.

Small style adjustments to tags and the editor text area.

Fix spacing between icons and text inside buttons, improve spacing on annotation cards, add placeholder for annotation creation textarea, adjust padding on tags.

Convert SCSS variables into hyphen case