Allow administrators to choose which merge strategies are allowed to be chosen

Would be great to be able to force developers to only be able to use certain types of merge strategies.

In our case we want to disable all types of merge strategies except for squash. Currently some developers miss this and choose the default "merge", creating a very unclean history for our master branch. It also makes it difficult to cherry pick hotfixes from master branch to our specific release branch.

Please provide the option to choose which merge strategies are available in drop down when merging PRs!