Add Union Base to ProgCode's "Featured Projects" Section

[PamelaJohn]

URL: https://unionbase.org/

Slack Button: http://progco.de/join

Description: The labor movement's social network and search engine, revolutionizing labor organizing.

Logo:

[stephenscapelliti]

Yonathan Micael and Larry Williams, Jr. of UnionBase provided additional logos to try for a better fit in the Projects section.
Note that a .psd file also was provided but is not supported in GitHub. It can be retrieved from the admin@progcode.org mail.