Add a new table issue_index to store the max issue index so that issue could be deleted with no duplicated index (#15599)

* Add a new table issue_index to store the max issue index so that issue could be deleted with no duplicated index

* Fix pull index

* Add tests for concurrent creating issues

* Fix lint

* Fix tests

* Fix postgres test

* Add test for migration v180

* Rename wrong test file name

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

models/fixtures/issue.yml

@@ -152,7 +152,7 @@
 -
   id: 13
   repo_id: 50
-  index: 0
+  index: 1
   poster_id: 2
   name: issue in active repo
   content: we'll be testing github issue 13171 with this.
@@ -164,7 +164,7 @@
 -
   id: 14
   repo_id: 51
-  index: 0
+  index: 1
   poster_id: 2
   name: issue in archived repo
   content: we'll be testing github issue 13171 with this.

models/fixtures/issue_index.yml

@@ -0,0 +1,24 @@
+-
+  group_id: 1
+  max_index: 5
+-
+  group_id: 2
+  max_index: 2
+-
+  group_id: 3
+  max_index: 2
+-
+  group_id: 10
+  max_index: 1
+-
+  group_id: 48
+  max_index: 1
+-
+  group_id: 42
+  max_index: 1
+-
+  group_id: 50
+  max_index: 1
+-
+  group_id: 51
+  max_index: 1