Factor out 520-byte value size limit

[JeremyRand]

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

• electrum-nmc/electrum_nmc/electrum/gui/qt/configure_dns_dialog.py

  Line 657 in 3a56875

  if usage > 520:

• electrum-nmc/electrum_nmc/electrum/gui/qt/forms/configurenamedialog.ui

  Line 139 in 3a56875

  <number>520</number>

• electrum-nmc/electrum_nmc/electrum/gui/qt/forms/dnsdialog.ui

  Line 1148 in 3a56875

  <string>/ 520</string>

• electrum-nmc/electrum_nmc/electrum/names.py

  Line 128 in 3a56875

  value_length_limit = 520

• 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.

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[ghost]

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