Use webpack+handlebars-loader for the Handlebars precompilation

[ChristophWurst]

Is your feature request related to a problem? Please describe.
The strict CSP in Nextcloud 15 forced us to precompile our templates. Currently this happens through a custom script.

Describe the solution you'd like
Use webpack and the handlebars-loader to precompile and package templates with the server bundle. This will keep all the building in one place and replaces our custom script with a de facto standard tool for this task.

Describe alternatives you've considered
Don't change this.

Additional context

• Requires Use npm for js depdencies, bundle with webpack #13156

cc @nextcloud/javascript @rullzer

[MorrisJobke]

• Requires Use npm for js depdencies, bundle with webpack #13156

This is in now.

Still doable for 16 or something for 17?

[rullzer]

Some is done for 16 rest we'll tackle in 17.