Offline receipt sending fields inconsistently handled

The backend contribution, event registration, and membership forms each offer the ability to email a receipt to the contact. However, this parallel behavior is handled independently for each form and is done inconsistently. This has caused a handful of issues:

1. The admin can specify receipt text for memberships and event registrations but not for contributions. @alicefrumin wrote a fix for this in PR 15605, but @bgm thought it required a Gitlab issue to discuss and @eileen was uncomfortable with any improvements to receipts until some code is cleaned up.

2. There is a regression in 5.21 where the backend new contribution form in "standalone" context (coming in cold, rather than from a contact) never allows you to send a receipt from the form. This appears to have been introduced by PR 15757 which, ironically enough, was an attempt to make the new contribution form work consistently with member, participant, and grant forms.

I think it will be challenging to resolve all of the ways the forms are similar but inconsistent, but the receipts are a clear problem and perhaps somewhere to start.