Website revamp

[fmaussion]

I have become quite unhappy with how our webpage looks like. In fact, the only thing I really like is the welcome screen, but otherwise there are several issues (see e.g. #53) which are know are solved by other templates.

@OGGM/oggm-devs if you know of someone with web / jekyll skills, let me know ;-) . Maybe we could hire this person for a proper revamp / rewrite from which we could start from.