RFR: 1547: Collect PR specific constants in single shared class

[Zhao Song]

There are numerous string literals and constants duplicated and spread around different bots. This includes markers (used in PR body and comments) for various bot functionality as well as label names.

In this patch,  I cleaned the constants in different bots according to this standard:

1. If a constant is used by multiple bots, it should be placed in bots.common.PullRequestConstants.
2. If a constant is used by different classes within a single bot, it should be placed in a single class within the bot, and other classes should import the constant.
3. If a constant is only used by one class within a single bot, it should be made private to that class.

Also, I have renamed the constants that were previously named in an inconsistent manner.

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Commit messages:
 - SKARA-1547

Changes: https://git.openjdk.org/skara/pull/1464/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1464&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1547
  Stats: 344 lines in 40 files changed: 65 ins; 26 del; 253 mod
  Patch: https://git.openjdk.org/skara/pull/1464.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1464/head:pull/1464

PR: https://git.openjdk.org/skara/pull/1464