Fix#5997.
If a push causes the patch/diff of a PR towards target branch to change, all existing reviews for the PR will be set and shown as stale.
New branch protection option to dismiss stale approvals are added.
To show that a review is not based on the latest PR changes, an hourglass is shown
* Possibility to not use whitelist but allow anyone with write access
* fix existing test
* rename migration function
* Try to give a better name for migration step
* Clear settings if higher level setting is not set
* Move official reviews to db instead of counting approvals each time
* migration
* fix
* fix migration
* fix migration
* Remove NOT NULL from EnableWhitelist as migration isn't possible
* Fix migration, reviews are connected to issues.
* Fix SQL query issues in GetReviewersByPullID.
* Simplify function GetReviewersByIssueID
* Handle reviewers that has been deleted
* Ensure reviews for test is in a well defined order
* Only clear and set official reviews when it is an approve or reject.
* extract actions on new pull request from models to pulls service
* improve code
* move code.gitea.io/gitea/modules/pull to code.gitea.io/gitea/services/pull
* fix fmt
* Rename pulls.go to pull.go
