RFR: 2000: Escape user handles in bug titles in pr body

[Zhao Song]

In this patch, the bot would escape `@` in the issue title in pr body.

I tried to escape `@` with `@`, but failed, the text would still mention the link.

So according to this [page](https://github.com/github/markup/issues/1168#issuecomment-494946168), I escape `@` with `@`.

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Commit messages:
 - SKARA-2000

Changes: https://git.openjdk.org/skara/pull/1548/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1548&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2000
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1548.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1548/head:pull/1548

PR: https://git.openjdk.org/skara/pull/1548