Extracting html templates into a library (#16946)

* Extracting html templates into a library

* Moving tests to the right place

* Fixing tests

* Addressing PR review comments

* Addressing PR review comments

* Replacing attomic with RWMutex

* Returning errors as channel for Templates watcher

* Address PR review comments

* Other small fixes

* Simplifying NewWithWatcher

* Addressing PR review comments

* Making error handling on rendering templates more robust

* Fixing tests

* Changing how we return errors

* Fixing shadow variables

* Addressing PR review comments

* Logging errors from the outside of sendNotificationEmail

* Fixing lock in shutdown

* Fixing the resource copy for commands tests temporary directories

* Removing unused import

* A couple of tiny fixes

testlib/resources.go

@@ -87,6 +87,7 @@ func getTestResourcesToSetup() []testResourceDetails {
 
 	var testResourcesToSetup = []testResourceDetails{
 		{root, "mattermost-server", resourceTypeFolder, actionSymlink},
+		{"go.mod", "go.mod", resourceTypeFile, actionSymlink},
 		{"i18n", "i18n", resourceTypeFolder, actionSymlink},
 		{"templates", "templates", resourceTypeFolder, actionSymlink},
 		{"tests", "tests", resourceTypeFolder, actionSymlink},