Add blank option to server IP-pool select; prompt for rule form

[vaibhavheda]

Summary

The IP-pool collection_select on the server form (and rule form) had no blank option, so once a pool was assigned, there was no UI path to clear it. The model already accepts a nil ip_pool_id — only the view
stood in the way.

• Server form: adds include_blank: "No specific pool (use rules / default)" (server pool is optional).
• Rule form: adds prompt: "Select an IP pool" (rule pool is required, so prompt is more appropriate than blank).

Test plan

• Picking the blank option in the server form persists servers.ip_pool_id = NULL.
• After unset, new QueuedMessage rows have ip_address_id = NULL.
• Toggle works in both directions.
• Rule form shows the prompt for new rules; existing rules render unchanged.

[vaibhavheda]

@willpower232 - Can take this forward please

Thanks