Feature/issue 261 export tags -Prune unrelated channel IDs from exported channel tags

[eddcomeau]

Summary

Prunes exported channel tag membership so channel exports only include the exported channel's ID in each tag.

Changes

• prune tag channel IDs in server-side export assembly
• prune tag channel IDs in client-side channel export
• update ChannelServletTest expectations for exported tag membership

Verification

• ChannelServletTest passes when run directly (It was edited as part of this commit)
• Rebuilt with Ant -DdisableTests=true
• Tested the UI (windows 10) by exporting a single channel, importing, and exporting a group, importing.

[github-actions]

Test Results

  111 files  ±0    214 suites  ±0   7m 22s ⏱️ + 1m 30s
  654 tests ±0    654 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 308 runs  ±0  1 308 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 11557b3. ± Comparison against base commit be1072d.