Allow filtering event log by related entries

[OmeGak]

Event log filtering by related entries relies on the new internal_data column added to the event.logs table. For now, the only logged events that use this new column are those related to registrations.

Screenshots

[ThiefMaster]

I noticed that we are not attaching the registration id when logging emails related to a registration. I thing doing it there too would be a good idea.

You'll have to add a way to pass-through internal data (maybe call it log_data there?) in send_email and _log_email; the rest should be straightforward.

[pferreir]

Thank you very much for the PR! 👍 I have left a few comments, nothing too critical, mostly about the naming of some stuff.

[plourenco]

Thanks! This works great! I only have a few minor suggestions below.

[OmeGak]

I noticed that we are not attaching the registration id when logging emails related to a registration. I thing doing it there too would be a good idea.

Done!