Remove the tooltip on the timestamp

Now just uses the default browser title functionality.
Always right align timestamps in cards

Move dropdown to the left of the timestamp to ensure timestamps are
always in line. Vertically align the baseline of the timestamp with the
username.
Remove dashed border on collapsed threads

It left a single pixel, which looked like a rendering glitch.
Clean up blockquote styles

* Remove cartoon quotes.
* Use a class rather than styling all blockquotes.
Correct expand/collapse icons on thread

Now collapsed items have a plus sign and expanded ones a minus. This
brings it inline with other tree view implementations such as those
found in Windows and Reddit where the icon represents the action
rather than the current state.
Drop tagit in favor of ngTagsInput
Move common form styles into mixins
Remove old tag-it css
Style the tags-input directive
Add new styles for read only tags
Remove the jQuery UI images and stylesheets
Remove the tag-it plugin and update the Karma config
Merge pull request #1414 from hypothesis/tags-input-styles

Styles the ngTagsInput
Merge pull request #1413 from hypothesis/drop-tagit

Drop tagit
Add markdown preview, add markdown help icon, and adjust styles for markdown output.