Resize event images and create miniature versions

Actually, images from events have a fixed height of 400px in the CSS but uploaded images have original sizes.

Also, I am not sure images are compressed on upload. Maybe this should be proposed as an opt-out option to save some space.

You can see how long is this event to load because I've uploaded a 1.4MB image:

Event: https://test.mobilizon.org/events/704191d8-fb4d-478a-9480-ef4581b374f0

Image: https://test.mobilizon.org/media/1b6795e3f76fac7739d3b0f04491ca0c8a9cc3a5dab6fa553ee4825ed0690ebb.png?name=fixture_by_sydebiased-d5zmicw.png