Refactor presentation file management and unique URL generation

Currently the way the presentation is sent to BBB is the following:

1. At meeting creation we decide on a unique URL (public URL) for BBB to hit moodle on to receive the presentation
2. Once the presentation is received from BBB the URL is not used anymore

We need to refactor slightly the file helper:

• To take into account the fact that now BBB only need access once to the file
• Make the code less repetitive and clearer to read
• Add more tests