"username" field ux cleanup and NIP05

[earonesty]

add the user types a "username", we append "@arcade.city" to it, showing that we are establishing a unique NIP05 identity

if the user types an "@" sign, we remove the arcade city, and assume the user already has a NIP05 name that they would prefer to use

we rename the field to "User unique name", so it's clear it must be unique

we register the name at arcade.city using a simple lamda that puts it in a db, and associates it with the pubkey (pay for usage only)

we expose an endpoint for "well known" nip05, so this works

done!