Calculate and submit translation's similarity ratio

Each translation submission will contain the similarity ratios calculated
based in the information the user has available on the screen. When the
Machine Translation feature is used, the MT suggestion will be used to
calculate the similarity against as well.

Fixes evernote/pootle#155.

Conflicts:
	docs/evernote_changelog.rst
	pootle/apps/pootle_app/assets.py
	pootle/apps/pootle_store/forms.py
	pootle/static/js/editor.js
	pootle/templates/editor/units/xhr_tm.html
Similarity: upgraded copy of levenshtein.js

Ensures non-string input doesn't crash.
Editor: simplified suggestion selector
Core: Sync pootle/templates/editor/units/xhr_tm.html with Evernote

Changes were introduced in 53280e4ced657a3fc88bf8337a2e7d2fcd65c6d3
We are throwing these away because we will go with Core's new
ElasticSearch based local TM.
Provide additional context information in TM results

Conflicts:
	pootle/templates/editor/units/xhr_tm.html
Editor: retrieve the text to be copied from data attributes

Relying on child selectors and extracting an element's text has been
troublesome because it ended up copying unwanted markup and whitespace.

Fixes evernote/pootle#190.

Conflicts:
	pootle/templates/editor/units/edit.html