support for a null send limit for unlimited

part of #144

app/models/server.rb

@@ -66,7 +66,6 @@ class Server < ApplicationRecord
   random_string :token, :type => :chars, :length => 6, :unique => true, :upper_letters_only => true
   default_value :permalink, -> { name ? name.parameterize : nil}
-  default_value :send_limit, -> { 100 }
   default_value :raw_message_retention_days, -> { 30 }
   default_value :raw_message_retention_size, -> { 2048 }
   default_value :message_retention_days, -> { 60 }
@@ -196,11 +195,11 @@ class Server < ApplicationRecord
   end
   def send_limit_approaching?
-    send_volume >= self.send_limit * 0.90
+    self.send_limit && (send_volume >= self.send_limit * 0.90)
   end
   def send_limit_exceeded?
-    send_volume >= self.send_limit
+    self.send_limit && send_volume >= self.send_limit
   end
   def send_limit_warning(type)

app/views/servers/admin.html.haml

@@ -11,7 +11,7 @@
       %fieldset.fieldSet.fieldSet--wide
         .fieldSet__field
           = f.label :send_limit, :class => 'fieldSet__label'
-          .fieldSet__input= f.text_field :send_limit, :autofocus => true, :class => 'input input--text'
+          .fieldSet__input= f.text_field :send_limit, :class => 'input input--text', :placeholder => "No limit"
         .fieldSet__field
           = f.label :allow_sender, "Allow sender header", :class => 'fieldSet__label'
           .fieldSet__input= f.select :allow_sender, [["No", false], ["Yes - can use Sender header", true]], {}, :class => 'input input--select'