Factor out 520-byte value size limit

The 520-byte value size limit shows up in a few places in the source:

https://github.com/namecoin/electrum-nmc/blob/3a5687512acc8505b344adcbacc75f24a8bd2fd2/electrum_nmc/electrum/gui/qt/configure_dns_dialog.py#L657
https://github.com/namecoin/electrum-nmc/blob/3a5687512acc8505b344adcbacc75f24a8bd2fd2/electrum_nmc/electrum/gui/qt/forms/configurenamedialog.ui#L139
https://github.com/namecoin/electrum-nmc/blob/3a5687512acc8505b344adcbacc75f24a8bd2fd2/electrum_nmc/electrum/gui/qt/forms/dnsdialog.ui#L1148
https://github.com/namecoin/electrum-nmc/blob/3a5687512acc8505b344adcbacc75f24a8bd2fd2/electrum_nmc/electrum/names.py#L128
Maybe more places?

These should be factored out, probably to a constant in names.py. The same might be necessary for the identifier size limit. Thanks to Zoltan Konder for noticing this.

namecoin / electrum-nmc

Factor out 520-byte value size limit #319