* Adding Tether as BTCPay Server Foundation Supporter
* Adding Tether to _BTCPaySupporters partial as well
* Modfying supporter_strike.svg to have white backgroundf or dark mode
* Modifying supporter_tether.svg to fit in the 150x100 box
* Centering Tether shape
The `img/readme` directory contains SVGs for the README, so that we can from now on use one markup for the supporters in all README files across our repositories.
With these, we could finally get rid of the table layout for the supporters section in the README. This will make it much easier to maintain those.
