#146 - adding login via social media

[Lee0z]

Added feature that allows users to login using social media accounts. For now it features Github, Facebook and Google.

• Social media login is available through login form:

  

• It is also translated:

  

Backend is done by @JakubKermes.

It should close #146.

[EwelinaSkrzypacz]

Conflicts here - please resolve them