support gatherings with > 19 participants

[brenzi]

As we are increasing the target gathering size to 15, it can happen that gatherings get assigned with 19 or even more people.

19 seems supported: https://github.com/encointer/encointer-wallet-flutter/tree/master/app/assets/avatars

but this is very tight. See probabilities here:
encointer/pallets#372

[clangenb]

@Eldar2021 Do you remember, where you got the avatars from? :)

[Eldar2021]

I don't remember much, but I'll try to find it on the internet.

[Eldar2021]

I'm glad I found it easy :)

https://www.svgrepo.com/svg/191396/lion

In this case I suggest to place avatars in the encointer-feed repository and use them with SvgPicture.network in the app. Why?

1. We are not releasing a new version just for a small avatar.
2. The size of our .apk/.ipa file will decrease slightly.

[clangenb]

Thanks a lot @Eldar2021! Out of laziness, I did not change the process of getting the avatar, although I do think that the apk size matters. On the other hand, we want to allow offline meetups, which is why we should have the avatars inside the apk.