Add a permission merge veto to facilitate showing vetoes to the PR author regardless of permission level

Problem

The merge button is not shown if you do not have REPO_WRITE permission.

This means none of the merge vetoes are visible to a pull request author if they do not have write access to the repository.

Use case

If a repository has a fork based workflow whereby PR authors only have REPO_READ on the target repository and a "trusted user" is allowed to perform the merge as they will have REPO_WRITE, the author will not be able to see any merge vetoes as the merge button will be hidden.

Proposed Solutions

• Make a permission merge veto to inform a user that they do not have the right permission set to merge this pull request. This would always show the merge button to everyone that sees the pull request but disable it for users that only have REPO_READ.
• Similar to the above, but instead of creating a separate merge veto, disable the button and add a message to the merge checks dialog to indicate the user does not have sufficient permissions to merge the pull request.