Composer - Move comment about the signatures on top to the right place
EHTMLEditor - Various fixes when editting the message as new

Don't insert the unnecessary new line into the body and always move the
caret to the beginning the of body. Also rename the is_from_draft
variable in composer to is_message_from_draft.
EHTMLEditorView - Correct the logic to get if the body is empty to avoid the unnecessary processing of it
EHTMLEditorView - Minor refactoring