Stash should expose the plugin.stash-scm-git.pullrequest.merge.strategy* setting in the UI

As suggested in STASH-2874, the need to have the Stash admin change the properties for all plugin.stash-scm-git.pullrequest.merge.strategy* options induces an unnecessary burden on the Stash admin and is not scalable.

Since Stash Admins, Project Admins and Repo Admins are not necessarily the same people and because the UI doesn't expose the setting for the preferred pull request merge strategy, teams will be unable to experiment easily with another pull request merge strategy and see if other merge strategies fit better their needs.

Please expose the following settings in the UI:

Per repository configuration (where "KEY" is the project key and "slug" is the repository slug):
plugin.stash-scm-git.pullrequest.merge.strategy.KEY.slug

Per project configuration:
plugin.stash-scm-git.pullrequest.merge.strategy.KEY

Optionally, the global configuration could be exposed, too:
plugin.stash-scm-git.pullrequest.merge.strategy

Initial suggestion was at:
https://jira.atlassian.com/browse/STASH-2874?focusedCommentId=703643&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-703643