Add default files for standard usecases

[Dagefoerde]

Together with the HS Hannover and Synergy Learning, we are planning to contribute the following enhancement:

Add default files for standard use cases. Specifically, during the creation of an activity instance, allow teachers to select whether the document is going to be a spreadsheet, a word document, or a presentation. Depending on the chosen type, initialize the editor with a template file of the respective type.

This option is presented in addition to the current option for uploading a template file.

As a default, the template files shipped with this plugin shall be blank files of an appropriate type. Administrators will be provided with the possibility to upload alternative template files.

(This is one of the tasks resulting from the general idea that I presented in #5.)

[logicexpertise]

I'd be interested in seeing the workflows around this feature