Add frontend to show message edit history.

Fixes #268.

Modified significantly by tabbott to:
* improve code cleanliness / repetition
* add missing translation tags
* move code into message_edit.js
* correspond with the new backend.
* not display the option for messages only topic-edited

zerver/tests/test_templates.py

@@ -76,6 +76,7 @@ class TemplateTestCase(ZulipTestCase):
             'zerver/stream_creation_prompt.html',
             'zerver/subscriptions.html',
             'zerver/tutorial_finale.html',
+            'zerver/message_history.html',
         ]
         unusual = [
             'confirmation/mituser_confirmation_email_body.txt',