MM-29726 Allow disabling link previews from certain domains (#16869)

* Add field to config model

Config option for disabling link previews for given domains.

* Refactor functions and corresponding tests

* Expand logic for link preview

Newly added isLinkAllowedForPreview function determines whether a link
should display a preview. It gets corresponding config values consisting
of comma separated domain values, normalizes them and checks for
matches.

* Create tests for link preview restriction

* Fix formatting issue

* Add test cases where images are expected

* Apply suggestions from code review

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Apply remaining code suggestions

* Add RestrictLinkPreviews value to telemetries

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Harrison Healey <harrisonmhealey@gmail.com>

model/config.go

@@ -300,6 +300,7 @@ type ServiceSettings struct {
 	EnablePostUsernameOverride                        *bool    `access:"integrations"`
 	EnablePostIconOverride                            *bool    `access:"integrations"`
 	EnableLinkPreviews                                *bool    `access:"site"`
+	RestrictLinkPreviews                              *string  `access:"site"`
 	EnableTesting                                     *bool    `access:"environment,write_restrictable,cloud_restrictable"`
 	EnableDeveloper                                   *bool    `access:"environment,write_restrictable,cloud_restrictable"`
 	EnableOpenTracing                                 *bool    `access:"write_restrictable,cloud_restrictable"`
@@ -407,6 +408,10 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 		s.EnableLinkPreviews = NewBool(true)
 	}
 
+	if s.RestrictLinkPreviews == nil {
+		s.RestrictLinkPreviews = NewString("")
+	}
+
 	if s.EnableTesting == nil {
 		s.EnableTesting = NewBool(false)
 	}