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/migrations/migrations.go

@@ -160,6 +160,8 @@ var migrations = []Migration{
 	NewMigration("add lfs lock table", addLFSLock),
 	// v53 -> v54
 	NewMigration("add reactions", addReactions),
+	// v54 -> v55
+	NewMigration("add pull request options", addPullRequestOptions),
 }
 
 // Migrate database to current version