RFR: 720: Incorrect usage instructions for label command

[Erik Helin]

Hi all,

please review this patch that improves the messages from the bots wrt to applied labels. For automatically applied
labels we now list the labels more prominently. This is done to bring more attention to lists that will be CC:d. When
no labels are automatically found then the message has been made a bit shorter by using a dropdown for revealing
applicable labels. Both messages now link to the `/label` pull request command documentation instead of describing it
in the messages.

Testing:
- [x] Updated a unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - 720

Changes: https://git.openjdk.java.net/skara/pull/842/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=842&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-720
  Stats: 30 lines in 2 files changed: 11 ins; 1 del; 18 mod
  Patch: https://git.openjdk.java.net/skara/pull/842.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/842/head:pull/842

PR: https://git.openjdk.java.net/skara/pull/842