ZO-4687: Store our own date_last_modified instead of relying on DAV #84

All 2 681 tests pass, 56 skipped in 8m 46s

2 files + 2 2 suites +2 8m 46s ⏱️ + 8m 46s
2 737 tests +2 737 2 681 ✅ +2 681 56 💤 +56 0 ❌ ±0
2 766 runs +2 766 2 710 ✅ +2 710 56 💤 +56 0 ❌ ±0

Results for commit 5a1cf69. ± Comparison against earlier commit 7e9562b.