Show first name and last name for users in network/admin users

[PeteMall]

We display the user's first name + last name in siteadmin users list-table but use the display name in the network admin users list-table.

Jane suggested using first name + last name in both places.

[nacin]

Looks good.

For another ticket, I guess: Noticing that these aren't translatable, in either table. Probably not a regression, but shouldn't this be %1$s %2$s with a translator's comment of Translators: %1 first name, %2 last name, for languages that flip these? I think that would also require a new sorting algorithm as well.

[PeteMall]

(In [16530]) Use first and last name in network admin users. Fixes #15528.