Some organization logos fail to render due to external image hosting

[rivaquiroga]

Several logo URLs in _data/organizations.yaml rely on external image hosting, which causes them to silently fail to render on the site.
For some organizations, (e.g. denbi, elixir, mozilla), we are currently storing locally their logos under /images/organizations/. We should do that for the rest, and then update the corresponding entries in _data/organizations.yaml to use local paths.

[rivaquiroga]

Thanks @richarddushime for spotting this issue!