Add review request api (#11355)

* Add review request api

* add : POST /repos/{owner}/{repo}/pulls/{index}/requested_reviewers
* Remove : DELET /repos/{owner}/{repo}/pulls/{index}/requested_reviewers
* fix some request review bug
* block delet request review by models/DeleteReview()

Signed-off-by: a1012112796 <1012112796@qq.com>

* make fmt

* fix bug

* fix test code

* fix typo

* Apply suggestion from code review @jonasfranz

* fix swagger ref

* fix typo

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

* fix comment

* Change response message

* chang response so some simplfy

* Add ErrIllLegalReviewRequest
fix some nits

* make fmt

* Apply suggestions from code review

Co-authored-by: silverwind <me@silverwind.io>

* * Add team support
* fix test
* fix an known bug

* fix nit

* fix test

* Apply suggestions from code review

Co-authored-by: zeripath <art27@cantab.net>

* update get api and add test

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: zeripath <art27@cantab.net>

models/fixtures/issue.yml

@@ -135,3 +135,15 @@
   is_pull: true
   created_unix: 1579194806
   updated_unix: 1579194806
+
+-
+  id: 12
+  repo_id: 3
+  index: 2
+  poster_id: 2
+  name: pull6
+  content: content for the a pull request
+  is_closed: false
+  is_pull: true
+  created_unix: 1602935696
+  updated_unix: 1602935696