Hide the first and last new line when editing a standalone macro

Right now if you write:

{{velocity}}
## velocity code here
{{/velocity}}

and edit this macro in WYSIWYG mode you'll get:

## velocity code here

in the macro content text area. Notice the first and last empty lines. They are used to format the wiki code and shouldn't be taken as part of the macro content. In order to be consistent with the normalized form of standalone macros we should hide the first and last new line when editing standalone macros and generate them if not present when saving the macro.