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/bare.tmpl

@@ -5,7 +5,7 @@
 		<div class="ui grid">
 			<div class="sixteen wide column content">
 				{{template "base/alert" .}}
-				{{if .IsRepositoryWriter}}
+				{{if .CanWriteCode}}
 					<h4 class="ui top attached header">
 						{{.i18n.Tr "repo.quick_guide"}}
 					</h4>