MixCloud oEmbed support

Adding support for MixCloud oEmbed
Added MixCloud oEmbed to oembed_providers_list
Merge branch 'develop' of https://github.com/diaspora/diaspora into develop
oembed_providers.yml added for custom (outside of ruby-oembed) oEmbed providers.
Merge pull request #4131 from Ruxton/patch-1

Refactoring oEmbed Providers & Adding MixCloud oEmbed support