Make the mobile menu CTA editable from the menu settings (develop)

[mlaetitia]

Related to T51ENG-2081

Proposed changes

• Make the mobile menu's CTA button editable via a "Mobile menu CTA" link field on the primary menu's settings; no field value means no button

Why are these changes being made?

• The CTA label/URL was hardcoded in the templates; editors should control it (including removing it) without a deploy

Testing instructions

• In Appearance → Menus (classic screen), set the "Mobile menu CTA" link on the primary menu and verify the mobile menu's bottom button uses it (including new-tab if checked)
• Clear the field and verify the button disappears
• Note: after deploy the button is hidden until the field is set on the menu