Change label of switch control in Page properties dialog

Given the outcome of the investigation into CHANNELMGR-2194: 
Label of the switch control should be 'Apply new template' (rather than 'Assign a new template' which it is now).

Changing the template is a one time thing; the template is applied once but if it (the template) is changed later, the page doesn't change along with it. 'Assign' suggests a (remaining) link between page and template that in reality is not there.