Restrict permission check on repositories and fix some problems (#5314)

* fix units permission problems

* fix some bugs and merge LoadUnits to repoAssignment

* refactor permission struct and add some copyright heads

* remove unused codes

* fix routes units check

* improve permission check

* add unit tests for permission

* fix typo

* fix tests

* fix some routes

* fix api permission check

* improve permission check

* fix some permission check

* fix tests

* fix tests

* improve some permission check

* fix some permission check

* refactor AccessLevel

* fix bug

* fix tests

* fix tests

* fix tests

* fix AccessLevel

* rename CanAccess

* fix tests

* fix comment

* fix bug

* add missing unit for test repos

* fix bug

* rename some functions

* fix routes check

templates/repo/issue/milestone_new.tmpl

@@ -4,7 +4,7 @@
 	<div class="ui container">
 		<div class="navbar">
 			{{template "repo/issue/navbar" .}}
-			{{if and .IsRepositoryWriter .PageIsEditMilestone}}
+			{{if and (or .CanWriteIssues .CanWritePulls) .PageIsEditMilestone}}
 				<div class="ui right floated secondary menu">
 					<a class="ui green button" href="{{$.RepoLink}}/milestones/new">{{.i18n.Tr "repo.milestones.new"}}</a>
 				</div>