Fix URL navigation and text alignment issues in inlineEditor.

inlineEditor's dragged text detection on <a href="..."/> elements would
fail to prevent the default action of navigating to the page after
opening the editor. The mouseup was too late to block this. To fix this
properly, we now register a click event that directly opens the editor
and prevents the default action.

There were also issues with sizing the editor on elements with a custom
text-align property. It wasn't able to find the ideal bounds for the
editor, and would result in a pretty small text field just big enough
for the original text. It now works around this by setting the
text-align property of the parent to "left" while doing the calcuations,
restoring it afterward.
Template tags from djblets_images could use Image module from PIL or Pillow, so it is not necessary to have strict requirement of this package.
Field extras_require was changed from list to dict. RULES-699.