CRM-19556 CiviMember: wording on "current" membership search

[agh1]

The work by @liedekef for searching memberships that are in "current" statuses is great. There are a couple of issues this addresses:

• The use of ts() with Active or Inactive as a variable means that the English word "Active" would be in there no matter what language. Of course, we could wrap "Active" with another ts(), but the translator would have no context for this. Sometimes it's better to repeat yourself if it involves translated strings.
• The is_current_member label is called "Current Membership" when editing membership types, so we should be consistent with that.

<rant>
Meanwhile, I happen to think the "current" term is terribly confusing in a couple of ways, but it's outside the scope of this fix. My concerns are:

• The obvious confusion that multiple statuses can be current while CiviCRM ships with a status named "current". The use of "Active" is much better in my mind; it just needs to be consistent across everything.

• The help text for the "current membership" field indicates that it marks whether someone is a member "in good standing". From that perspective, a member in a grace period should not be considered "current": they owe a renewal.

  On the other hand, the "current membership" field determines whether a renewal extends the old membership term (with the same start date and an end date extended by a term) or sets a new start date and calculates the end date from there. From this perspective, a member in a grace period should be considered "current": when they renew, there should be no lapse.

  This is an issue for another day, but those concepts should be split: one field should indicate which statuses are "active" from the perspective of being paid-up and/or receiving member benefits, and another field should indicate whether renewing a membership in that status should leave a lapse.

</rant>

---

• CRM-19556: Allow to search on active membership

[eileenmcnaughton]

This does only touch wording and Andrew has thought through the correct approach carefully. It is an rc regression, hence merging to the rc.