Use posterior in brms

[paul-buerkner]

Our new posterior package (https://github.com/stan-dev/posterior) is now on CRAN and we can now use it in brms as well. Among others, this will include using convergence diagnostics and summary methods from posterior as well as adding methods to extract posterior samples in all the different draws formats of posterior.

[paul-buerkner]

Closed via #1210