Use semi-expiration in Manage Names tab

[JeremyRand]

Is your feature request related to a problem? Please describe.

The Manage Names tab shows a countdown to expiry. This may give the user the impression that their name does not need renewal yet, when in reality it does due to semi-expiration.

Describe the solution you'd like

Switch the Manage Names tab to show a countdown to semi-expiry instead.

Describe alternatives you've considered

N/A.

Additional context

N/A.

[JeremyRand]

Blocked by https://github.com/namecoin/namecoin-core/issues/532