[Mindmaps] Add mindmaps for the course contents

[shorodilov]

• Add Django introduction mindmap doc (Add Django introduction presentation #221)
• Update db normalization mindmap (Database normalization #202)
• Add ACID mindmap (ACID #206)
• Add functions mindmap (Functions #216)
• Update functions mindmap (Functions #216)
• Add DTL mindmap ((DTL) Django template language #89, Add Django templates presentation #224)
• Add Django models mindmap (Django models #90, Add Django models presentation #225)
• Update Django models mindmap (Django models #90, Add Django models presentation #225)
• Update Django models mindmap (Django models #90, Add Django models presentation #225)
• Add RDBMS relationships mindmap