[Integrated] RFR: Add pull request prune functionality

[Robin Westberg]

Changeset: e75c0b06
Author:    Robin Westberg <rwestberg at openjdk.org>
Date:      2019-11-12 13:18:36 +0000
URL:       https://git.openjdk.java.net/skara/commit/e75c0b06

Add pull request prune functionality

Reviewed-by: ehelin, kcr

- bots/bridgekeeper/src/main/java/org/openjdk/skara/bots/bridgekeeper/BridgekeeperBot.java
! bots/bridgekeeper/src/main/java/org/openjdk/skara/bots/bridgekeeper/BridgekeeperBotFactory.java
+ bots/bridgekeeper/src/main/java/org/openjdk/skara/bots/bridgekeeper/PullRequestCloserBot.java
+ bots/bridgekeeper/src/main/java/org/openjdk/skara/bots/bridgekeeper/PullRequestPrunerBot.java
- bots/bridgekeeper/src/test/java/org/openjdk/skara/bots/bridgekeeper/BridgekeeperBotTests.java
+ bots/bridgekeeper/src/test/java/org/openjdk/skara/bots/bridgekeeper/PullRequestCloserBotTests.java
+ bots/bridgekeeper/src/test/java/org/openjdk/skara/bots/bridgekeeper/PullRequestPrunerBotTests.java