Revert "Unblock contact by clicking on the `blocked' tag"

This reverts commit 8dc0bc0200b9cc7d4b93f9cab178b74e5a0da26e.

Conflicts:
	src/main/java/eu/siacs/conversations/entities/Account.java
	src/main/java/eu/siacs/conversations/entities/Contact.java
	src/main/java/eu/siacs/conversations/entities/ListItem.java
	src/main/java/eu/siacs/conversations/ui/BlockContactDialog.java
	src/main/java/eu/siacs/conversations/ui/ContactDetailsActivity.java
	src/main/java/eu/siacs/conversations/ui/ConversationActivity.java
	src/main/java/eu/siacs/conversations/ui/StartConversationActivity.java

See discussion on pull #796