Unify logos

There are 2 files with the haskell logo:

- https://github.com/haskell-infra/hl/tree/master/static/img
- https://github.com/haskell-infra/hl/blob/master/design/logo.svg

They are however slightly different 🤦‍♂️.
I guess the standalone one should be adapted to the wordmark, as it's the "more" official one as it's currently up center on haskell.org