Add Pull Request merge options - Ignore white-space for conflict checking, Rebase, Squash merge (#3188)

* Pull request options migration and UI in settings

* Add ignore whitespace functionality

* Fix settings if pull requests are disabled

* Fix migration transaction

* Merge with Rebase functionality

* UI changes and related functionality for pull request merging button

* Implement squash functionality

* Fix rebase merging

* Fix pull request merge tests

* Add squash and rebase tests

* Fix API method to reuse default message functions

* Some refactoring and small fixes

* Remove more hardcoded values from tests

* Remove unneeded check from API method

* Fix variable name and comment typo

* Fix reset commit count after PR merge

models/repo.go

@@ -427,6 +427,11 @@ func (repo *Repository) MustGetUnit(tp UnitType) *RepoUnit {
 			Type:   tp,
 			Config: new(ExternalTrackerConfig),
 		}
+	} else if tp == UnitTypePullRequests {
+		return &RepoUnit{
+			Type:   tp,
+			Config: new(PullRequestsConfig),
+		}
 	}
 	return &RepoUnit{
 		Type:   tp,