membership#9 - show user a notification on back-office renewals

[MegaphoneJon]

Overview

When renewing a membership via the back-office form, no on-screen notification appears. This is technically a regression but appears to be quite old, so no need for urgency.

Before

No notification appears.

After

Notification appears.

Technical Details

The only functional change is removing the two return statements and removing the sentence about the end date (per Eileen's request to align with PR #12583). I additionally extracted the renewal message into its own method so that I could extend the tests to cover it.

Comments

The screenshot above is from before I removed the sentence about the end date. This is also a redo of PR #13553.

https://lab.civicrm.org/dev/membership/issues/9

[civibot]

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[eileenmcnaughton]

test this please

[eileenmcnaughton]

looks good