Add logos for each group and channel

[DenisIzmaylov]

Generally every Group and Channel has a logo image. We can copy it and just past in Markdown Editor on GitHub (yes, it works).

[DenisIzmaylov]

Up!

[sergeysova]

Any progress here? @A-gambit

[lgg]

@DenisIzmaylov i think it's useless. Only text is more pleasant for reading.
e.g. in this repo https://github.com/mr-mig/ru-tech-chats it looks awful