Install devise invitable

Devise invitable gem is required to invite users.

app/views/devise/invitations/edit.html.haml

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+.container
+  .row
+    .col-md-6.col-md-offset-3
+      .panel.panel-default
+        .panel-heading
+          %h3.panel-title
+            %h2= t 'devise.invitations.edit.header'
+        .panel-body
+          = form_for(resource, as: resource_name, url: invitation_path(resource_name), html: { method: :put }) do |f|
+            = render 'devise/shared/error_messages', resource: resource
+            = f.hidden_field :invitation_token, readonly: true
+            - if f.object.class.require_password_on_accepting
+              = f.label :username
+              = f.text_field :username, class: 'form-control'
+              = f.label :password
+              = f.password_field :password, class: 'form-control'
+              = f.label :password_confirmation
+              = f.password_field :password_confirmation, class: 'form-control'
+            %br
+            = f.submit t('devise.invitations.edit.submit_button'), class: 'btn btn-default btn-primary'

app/views/devise/mailer/invitation_instructions.html.erb

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+<p><%= t("devise.mailer.invitation_instructions.hello", email: @resource.email) %></p>
+
+<p><%= t("devise.mailer.invitation_instructions.someone_invited_you", url: root_url) %></p>
+
+<p><%= link_to t("devise.mailer.invitation_instructions.accept"), accept_invitation_url(@resource, invitation_token: @token) %></p>
+
+<% if @resource.invitation_due_at %>
+  <p><%= t("devise.mailer.invitation_instructions.accept_until", due_date: l(@resource.invitation_due_at, format: :'devise.mailer.invitation_instructions.accept_until_format')) %></p>
+<% end %>
+
+<p><%= t("devise.mailer.invitation_instructions.ignore") %></p>

app/views/devise/mailer/invitation_instructions.text.erb

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+<%= t("devise.mailer.invitation_instructions.hello", email: @resource.email) %>
+
+<%= t("devise.mailer.invitation_instructions.someone_invited_you", url: root_url) %>
+
+<%= accept_invitation_url(@resource, invitation_token: @token) %>
+
+<% if @resource.invitation_due_at %>
+  <%= t("devise.mailer.invitation_instructions.accept_until", due_date: l(@resource.invitation_due_at, format: :'devise.mailer.invitation_instructions.accept_until_format')) %>
+<% end %>
+
+<%= t("devise.mailer.invitation_instructions.ignore") %>