We convert the following elements to use a class instead of
id for accessing them across the codebase:
* markdown_preview
* undo_markdown_preview
* markdown_preview_spinner
* message_edit_content
* preview_content
Converted them together since changes to one impacted the other in
some modules like click_handlers.
Also, added a function in rows to get `message_row`.
This is both more correct and also fixes this element having had a
name very similar to message_control_button, which refers to an
element in the message_controls section of a rendered message.
We move compose.html to compose.hbs file while keeping
`#compose` still in `home.html` as a hanger
where append rest of the elements.
This will provide us with two benefits:
* We could share common elements between message_edit_form and
compose.
* We can insert compose directly in any element. We may decide to
do it for recent topics.
Aman Agrawal