Allow users to add shortcuts for manually installed applications

We show app shortcuts on the front page of Plinth and in the Android app. Users who install some applications manually on their FreedomBox would want to see them listed alongside the default applications that come with their FreedomBox. This feature has more relevance in community deployments.

Create a directory called /var/lib/plinth/customization. A file called shortcuts.json will be expected in this directory which will be appended after verification to the output of plinth/api/1/shortcuts. The icons in shortcuts.json should point to a directory called themes/default/icons in the customization directory.

This structure is to allow further customization of a FreedomBox instance. More themes can be added. Icons are placed in the customization directory instead of in plinth's static directory to avoid overwrites and allow plinth apps to be self-contained modules in the future.

Plinth's shortcuts api and frontpage view check for the existence of additional shortcuts and serve them if they exist.