luci-mod-network: dhcp.js: improve behavior with default configuration

luci-mod-network: dhcp.js: improve behavior with default configuration

 - Hide anonymous section title if only one instance is present
 - Hide section remove button if only one instance is present
 - In case multiple sections are present, label the first and
   subsequent anonymous sections as `Default instance` and
   `Unnamed instance #N` respectively
 - Label named sections as `Instance "XXX"`
 - Label delete buttons as `Remove instance "XXX"`,
   `Remove default instance` or `Remove instance #N` depending on whether
   the associated instance is the first anonymous one, a subsequent
   anonymous one or a named one
 - Label add button as `Add server instance`
 - Add placeholder text `New instance name…` to the section name input

Ref: https://forum.openwrt.org/t/x/194048
Signed-off-by: Jo-Philipp Wich <jo@mein.io>