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 nameā¦` to the section name input
Ref: https://forum.openwrt.org/t/x/194048
Signed-off-by: Jo-Philipp Wich <jo@mein.io>