Break-up menu template into sub-templates

MR !15 creates a new template for the subnav toggle icon, i.e. the SVG and uses that instead of defining the SVG within the menu template. (This does nothing to address a template for the menu toggle as a whole.)

Thanks! I will open a follow-up to break out the menu toggle button, since this issue was scoped with two intents.

Automatically closed - issue fixed for 2 weeks with no activity.