RFR: 1853: Make it possible to disable merge PRs for a repository [v2]

[Zhao Song]

> In this patch, PR bot has been added with the capability to control whether merge-style pr is allowed in specific repositories.
> 
> By default, merge-style pr is enabled.
> 
> However, if it is desired to disable merge-style pr in a repo, the configuration can be added to the PR bot configuration as follows:
> 
> {
>   "pr": {
>       "repositories": {
>           "repo1": {
>                "merge": false
>           }
>        }
>   }
> }
> 
> 
> If merge-style pr is disabled and the title of the pr matches the pattern of merge-style pr, the pr bot would reply a warning message to the user and prompt for the next steps.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  Update bots/pr/src/main/java/org/openjdk/skara/bots/pr/CheckWorkItem.java
  
  Co-authored-by: Erik Joelsson <37597443+erikj79 at users.noreply.github.com>

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1499/files
  - new: https://git.openjdk.org/skara/pull/1499/files/ab57a13a..05244e22

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1499&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1499&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1499.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1499/head:pull/1499

PR: https://git.openjdk.org/skara/pull/1499