tasks API function moved to api_app
Removed unused template args
FileStash is rendered in Javascript now. It lacks distribute & delete actions though.

Also, the view templates are moved out to separate files; now they get included into the main file through Jinja2 include statements.
Simplified SendorTasksView to always do a full reconstruction of the view whenever elements are added/removed from the underlying collection. This is worse for performance but should not pose a problem for our use cases.
FileStash has a fully-operational Delete action.
Added file distribution to the UI again. The application is now fully operational. Removed some unused html files.

Some things are not handled very nicely;
- some state is kept in the DOM tree (there is logic which makes the "Distribute!" button get greyed out for 3 seconds)
- There are non-unique IDs in the DOM tree ('distribute-submit' is one example) which is not recommended
- The template for StashedFileView is very large and has the logic for showing distribution targets inlined
- There is a 10-second periodic refresh of all models. This should ideally be converted to a Websocket-based communications mechanism instead.