Adds a new field in all posts apis to return an array containing the reason why the post is muted

fixes #219

There are 5 muted reasons:

• 'MUTED_COMMUNITY_MODERATION': 0
• 'MUTED_COMMUNITY_TYPE': 1
• 'MUTED_PARENT': 2
• 'MUTED_REPUTATION': 3
• 'MUTED_ROLE_COMMUNITY': 4

tests:

• MUTED_COMMUNITY_MODERATION
• MUTED_COMMUNITY_TYPE
• MUTED_PARENT
• MUTED_REPUTATION
• MUTED_ROLE_COMMUNITY
• both MUTED_PARENT and MUTED_COMMUNITY_TYPE
• list muted reasons enum