Merge branch 'email-blacklist' into 'develop'

Add email blacklist, fixes #1404

Closes #1404

See merge request pleroma/pleroma!2837

diff --cc CHANGELOG.md
Simple merge

diff --cc config/config.exs
Simple merge

diff --cc config/description.exs
Simple merge

diff --cc docs/configuration/cheatsheet.md
Simple merge

diff --cc lib/pleroma/user.ex
Simple merge

diff --cc test/user_test.exs
Simple merge