Update social section with federated/decentralized social networks

[johnjago]

Since creating this list, there has been a proliferation of social networks that are part of the fediverse.

https://en.wikipedia.org/wiki/Fediverse

The social section should be updated to include the best of these, and those that are not open source should be removed. Even if the closed source social networks are ad-free, the model promoted by the decentralized is more sustainable and likely to be "awesome" for a long period of time.

Here is the proposed list of social networks to add:

• Manyverse
• diaspora*
• friendica
• PixelFed (use this instance: https://pixelfed.social/)

Remove MeWe, quitter.se (offline as of 2018-11-18)