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/fixtures/repo_unit.yml

@@ -30,7 +30,7 @@
   id: 5
   repo_id: 1
   type: 3
-  config: "{}"
+  config: "{\"IgnoreWhitespaceConflicts\":false,\"AllowMerge\":true,\"AllowRebase\":true,\"AllowSquash\":true}"
   created_unix: 946684810
 
 -
@@ -51,7 +51,7 @@
   id: 8
   repo_id: 3
   type: 3
-  config: "{}"
+  config: "{\"IgnoreWhitespaceConflicts\":true,\"AllowMerge\":true,\"AllowRebase\":false,\"AllowSquash\":false}"
   created_unix: 946684810
 
 -