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/wiki/start.tmpl

@@ -6,7 +6,7 @@
 			<span class="mega-octicon octicon-book"></span>
 			<h2>{{.i18n.Tr "repo.wiki.welcome"}}</h2>
 			<p>{{.i18n.Tr "repo.wiki.welcome_desc"}}</p>
-			{{if and .IsRepositoryWriter (not .Repository.IsMirror)}}
+			{{if and .CanWriteWiki (not .Repository.IsMirror)}}
 				<a class="ui green button" href="{{.RepoLink}}/wiki/_new">{{.i18n.Tr "repo.wiki.create_first_page"}}</a>
 			{{end}}
 		</div>