Store multiple images with different sizes for different purpose for each event

[LukasKalbertodt]

Currently we only store one image per event which is called thumbnail. This is by default just the first attachment with flavor player+preview. In Opencast's default workflows, thats a 720p image. We use that for thumbnails and since #710 also for the player. This is not ideal:

• 720p as player image is too small for higher resolution screens. Ideally we would have a few different resolutions for that: 720p, 1080p, 1440p, 2160p or something like that.
• 720p is often too large for thumbnails. It would be nicer to have smaller images, but also a number of different sizes so that the user's browser can download the most fitting one.

This issue touches different areas:

• Opencast should generate more useful images by default (Provide better and more thumbnail images by default opencast/opencast#3753)
• The Opencast Tobira module needs to be smarter in choosing images and send out multiple via the harvest API
• Tobira needs to store multiple images and use the most fitting one for each purpose.