Return a reason why a post is muted

Right now a post can be muted (bridge_api_object has grey to true) for those reasons:

• user has less than 1 rep
• The post is muted (by a community mod)
• User role is muted (in a community)
• User posted in a type 2 or 3 community when they didn't have the right to (non-member posting in an member only community for instance)
• The post is a comment and it's parent is muted

This can get confusing for users "why am I muted here ?" and some front ends may decide to honor "grey" in different ways or have it up to the user (eg: I want to see posts from minus 1 rep).

This is especially important because new community features are in the pipeline that will leverage muted as a way to enforce rules (eg: community-wide beneficiaries requirements)

The fix would be a new field on top of grey for backward compatibility called muted_reasons which will return basically an array of enums for all the reasons why a post is muted (as a single post can violate multiple rules).

so the returned enum array would be like [1,3] corresponding to like [MUTED_COMMUNITY_TYPE, MUTED_COMMUNITY_MOD]