Error reporting, group dates and steadier training plans

Added

• Failures get reported, and the screen stays usable. The dashboard and the mobile app send their own errors to Artim Stacktrace, and a view that breaks now shows a fallback instead of going blank.
• The app reports which screens are used. Screen views in the mobile app are tracked and sent to Artim Dash.

Improved

• Editing a training plan. An exercise change still saves when the exercise it refers to is gone, missing values fall back to defaults, and the plan reloads after an edit instead of showing the old state.
• Creating a group asks for everything it needs. Group creation checks that all fields are filled and that the dates are valid before it accepts them.

Fixed

• Group meeting dates are now normalised to Berlin time, so the day a meeting lands on no longer depends on the timezone it was read in.
• A user who belonged to no organisation could pass an authorization check that should have stopped them.
• The same group could show up twice in a user's group list.
• Default profile pictures now come from a new avatar service, and an error from that service no longer goes unhandled. Profile images are stored as PNG.
• Payment processing read its API key from the wrong setting, and now picks the key up at the point where a payment is created.