Keep label and disabled state of transclusion dialog apply button in sync

* Use "Insert template" when adding a single template
* Use "Insert transclusion" when adding a multi-part template
* Use "Apply changes" when working with an existing tranclusion
* Use "Loading..." and disable while waiting for template data
* Disable when the transclusion has only a template placeholder

Bug: 50998
Change-Id: Ib2fb3d8711ed6d3ef41cc0db55740c95394dd3f9
Merge "Keep label and disabled state of transclusion dialog apply button in sync"