Add i18n-demo plugin

[tomasr8]

This is a plugin intended for the i18n demo instance. It does two things:

• All emails are redirected to the user who sent them (avoids spamming random people and lets you check email translations)
• Adds a new Clone button to the example events. When clicked, the corresponding event is cloned into your own personal subcategory and you get management rights to it. This way, you can see the management area without accidentally modifying the example events.

[tomasr8]

Note to self: email interception fails when the email is being sent from a celery task (e.g. data export). This is because at that point we're outside the request and don't have access to session.user anymore so we don't know who to redirect the email to

[tomasr8]

Note to self: email interception fails when the email is being sent from a celery task (e.g. data export). This is because at that point we're outside the request and don't have access to session.user anymore so we don't know who to redirect the email to

This should be 'fixed' now. We now substitute the recipient only when we have the request context which is most cases when sending an email. Two exceptions are data export where the recipient is the same user anyway, and reminders but hopefully people won't abuse this.

Otherwise this should be ready.