Make the location of "ui_results" configurable

[ikselven]

This PR makes the location of the directory ui_results configurable.

To achieve this, a new setting called RESULTS_DATA_DIR has been introduced in settings.py. It should contain the path to the ui_results directory, the directory where the Datacube UI saves its created product files into. Hardcoded paths that referred to ui_results have been replaced with a reference to this new setting.

Further adjustments have been made so that the paths to task result files are not stored as absolute paths in the database. With this PR, they are now relative to the configured RESULTS_DATA_DIR.