Think of better name for presences of unums

[jstuyts]

Unums have presences, but there is also a presence server. These are different kinds of presences (unum representations/views and whether a user is online respectively), so it is better to use different terms to avoid confusion.

Suggestions for unum presences:

• Presunce
• Ubeity (very uncommon word, so likely not known by many people)