PLATFORM-240 Enforce coding conventions for mustache templates

Several shared templates used a non-standard naming convention that
required extraneous boilerplate to use from controllers (specifically,
`$response->setTemplateEngine` and `$response->setTemplatePath`), and
made the mapping from controller action to template filename less
obvious. Rename these files to the expected filename and further reduce
boilerplate for fully-mustachioed extensions by adding support for
`DEFAULT_TEMPLATE_ENGINE` as a controller-level constant that defaults
the rendering engine to the defined value.

@owend @lizlux @kenkouot 

Template JS files regenerated via 
```bash
find . -name Gruntfile.js -not -iwholename "*node_modules*" -exec bash -c 'cd $(dirname {}) && npm install && grunt' \;
```