RFR: 905: Github UI code markdown can fail to get copied correctly into email

[Robin Westberg]

When filtering out certain markdown for sending plain-text emails, only remove multiline code formatting block markers and retain inline ones.

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Commit messages:
 - Only remove multi-line code markers

Changes: https://git.openjdk.java.net/skara/pull/1112/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1112&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-905
  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1112.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1112/head:pull/1112

PR: https://git.openjdk.java.net/skara/pull/1112