Align event images for the Community Calls

[spier]

I noticed that the images for the Community Calls on the Events page did not have the same size.
This lead to the other content below the image (title etc) to be misaligned. See screenshot.

This PR fixes that issue for the existing events.

While at it, I also made various other changes in the same area of the code:

• Re-create all all event images for the Community Calls (CCs) to 960x540
• Reduce the file size of all image, to make the page load faster. (we are loading 8 images just for the CCs, so the images got to be reasonably small)
• Rename images to form meetup-2021-05-04.jpg
• Consolide filename extension to .jpg (came in various froms before)
• Rename community events for file format meetup-YYYY-mm-dd.md, to bring them into natural order.
• Remove date info from the beginning of the event description. When the event has already taken place, the date information isn't that helpful for readers anymore. Also the date info is already available in the image of the title slide.
• Removing utm campaign info from one eventbrite link (we don't have that info for all other CCs)
• Remove featured: false as that is the default

[spier]

Here how the resulting events page looks like.

[claredillon]

Thanks so much for this Sebastian!

[spier]

Thanks for review and merge!

I think this is a great improvement already.

There are still some other UI glitches in this area that could still be improved:

1. Bring the images of the Summits to the same size, or find other ways to make sure that event title etc are aligned
2. How to align the text if the title breaks into 2 rows?
3. How to align the Open/Watch buttons horizontally? Maybe that would get fixed together with (2) automatically?
4. How to prevent repetitive intros like "Join us for an InnerSource Commons Community Call ..."