[feature]user can invite others via app contact
Merge pull request #83 from peterzd/invite_user_page

[feature]user can invite others via app contact
[feature]add InvitationMailer to send invitation email
[feature]user can invite others via email
Merge pull request #84 from peterzd/email_invite

Email invite
Merge pull request #85 from peterzd/dev

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