Fix permission check on Contribution form, clarify underlying functions

Fix permission check on Contribution form, clarify underlying functions

This is a reviewer response to https://github.com/civicrm/civicrm-core/pull/22951

In that PR it seeks to set UserID to something derived from a function that is doing
a lot of work. I dug into that function which someone (ahem) wrote many years ago
and teased out some of the underlying chunks & cleaned up the variable names for clarity
along with a doc block cleanup.

I decided that rather than set _userID in contributionBase we should remove it
(it is only referred to in 4 places) and call the more specific function at each usage.

Otherwise we change the meaning of userID that is used somewhat differently later.

Also I rather prefer functions that will perform the same whenever called over relying
on a variable having been set correctly the first time