Simplify the tags input styles

* Simplify the tags, no gradients or drop shadows.
* Use less padding to make them smaller.
* Remove the background color so they adopt the input background. This
  works nicely when the field loses focus.
* Increase the hit area of the delete button. It now takes up the entire
  right side of the tag.
Merge pull request #1424 from hypothesis/tweak-edit-tag-styles

Simplify the tags input styles