Export session participants

[rbento1096]

As an admin I'd like to be able to download the details about the participants for each session so I can share it with the speakers.

This data is saved on the registrations table and the most useful one is the name of the participant his esn country (if existent)

For this feature I'm thinking of 2 ways this can be processed:

• as a single call to the back-end that will scan all session registrations (at this time they are 5K+) and generate an excel with all data
• Add a button to the front-end that works as the current excel export of the users page (we can query the registrations database by sessionIdto get all users registered in a session). This would be simpler but would require more manual work from the organizers. Excel should have a name = session.code or session.name if code is not defined.

Ideally we could have both.

[uatisdeproblem]

I implemented both the proposed scenarios.

The feature is online for testing in the development environment. Note: since there aren't many registrations in the dev environment, I tested by registering my user to "How I Enforced my Inclusion in an Unfriendly Environment" (random session). It will also need to be tested with higher volumes of data in the production environment.

A few notes:

• Since the session data is repeated for each registration row in the spreadsheet, I added just the main data; feel free to add any more columns you like.
  
• For sessions that don't have any registration, I added a row anyway (with empty data for the participants' columns); if you prefer not to include the sessions without registrations in the spreadsheet, we can change it.
• The export is available only to users with "canManageContents" permissions.

The feature is available in three areas of the app:

1. The Manage menu (download ALL sessions registrations):
   

2. The sessions list, with a "current session" selected:
   

3. The detail of a session:
   

I will now open the PR.

[rbento1096]

It is working perfectly with production data as well!
We'll keep the rows for sessions without registration, can be useful for the programme team as well.

Thanks a lot!