Add host reservations form UI improvements

I have some ideas on how we can improve the host reservation form added in #720 (closed) .

• Autofill the IP input with the subnet prefix
• Use masked input for IP
• Add validation labels on input in addition to the red border (the red border doesn't describe what is wrong)
• Add a refresh button to the host reservation list
• Put the freshly added host reservations on the list but grayed them until fetched from the database. I need ~1 minute to fetch the reservations on my local setup. At this time, I started thinking that something had gone wrong.