This adds a class `rendered_markdown` for all the elements which have
rendered markdown content; This is done to add different styles for
rendered content in day mode and night mode.
Also replace the element selectors from CSS to use the class.
The message_edit_history UI was incorrectly inheriting its content
div's structure from message_edit_content, i.e. the form for editing
message content, and not message_content, i.e. the class for rendering
the content of messages in the message feed (which we also use for
drafts).
Fix this by changing the inheritance, while also adding a (currently
unused) class for any future customizations.
Fixes: #5629.
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
Vaibhav
Nikhil-Vats
Anders Kaseorg
Amadeo234
Cynthia Lin
Jack Zhang
Aditya Bansal
Kartik Maji