RFR: 2042: Allow bulk requests with /approval request command

[Zhao Song]

As some users requested, we will allow bulk requests with `/approval` request command.

If an id is not specified in the `/approval` command, the bot will generate a maintainer approval request for all associated bugs.

This also applies to the `/approval cancel` command, which will cancel the maintainer approval request for all associated bugs.

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Commit messages:
 - SKARA-2042

Changes: https://git.openjdk.org/skara/pull/1565/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1565&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2042
  Stats: 129 lines in 3 files changed: 43 ins; 56 del; 30 mod
  Patch: https://git.openjdk.org/skara/pull/1565.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1565/head:pull/1565

PR: https://git.openjdk.org/skara/pull/1565