E-mail notification upon sharing

It would be a good feature to allow users to globally enable notification when something is shared with them.

As @philip.abbet pointed out, it would be good if e-mails could be summarized in case a lot has been just shared with a person (e.g., when sharing an experiment, this normally includes algorithms, dataformats, the toolchain, libraries, etc). At this point, it would be best to receive a single e-mail listing all objects from the last sharing actions.

We should also think about the situation where various experiments are shared at once and avoid the e-mail spam once more on this case.

This feature could also have a hook to our to-be-implemented "activity" tab.