User must receive mailers when appropriate

[a17levine]

including but not limited to

• When user signs up, welcome to flashbang
• When a user posts an item, thanking them for their post
• When another user posts an item with a tag that the user getting the email is following
• When a post owner receives a comment on his/her post, unless it's the owner's comment
• Followers of a post get notified when the owner comments
• When a post owner receives an offer
• When an offer is accepted (post owner, offerer, all other offerers)
• When a user flags a post mail is sent to admins