Move hard-coded off-line receipt subjects from PHP files into templates so users can adjust as needed

Rationale
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Currently the "subject" text for most receipts / confirmations are set in template files - so they're easy to modify if needed. However, offline contribution receipts, offline membership signup and offline event registration share a single tpl file (templates/CRM/Contribute/Form/OfflineReceiptSubject.tpl) with hard-coded subjects passed in from the 3 PHP files.

Implementation
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1. Create separate offline subject template files for offline event registration and membership signup. Move hard-coded subject from PHP files into each tpl file.

2. Assign additional useful variables to the templates so users can add more info by modifying tpl files:

• for event registration, assign event title ($title)
• for membership, assign membership type ($membershipType)