MOBILE-3320 course: Improve link button icons in course page

src/addons/mod/lti/services/handlers/module.ts

@@ -78,7 +78,7 @@ export class AddonModLtiModuleHandlerService implements CoreCourseModuleHandler
                 CoreNavigator.navigateToSitePath(AddonModLtiModuleHandlerService.PAGE_NAME + routeParams, options);
             },
             buttons: [{
-                icon: 'link',
+                icon: 'fas-external-link-alt',
                 label: 'addon.mod_lti.launchactivity',
                 action: (event: Event, module: CoreCourseModule, courseId: number): void => {
                     // Launch the LTI.

src/addons/mod/url/services/handlers/module.ts

@@ -107,7 +107,7 @@ export class AddonModUrlModuleHandlerService implements CoreCourseModuleHandler
             },
             buttons: [{
                 hidden: true, // Hide it until we calculate if it should be displayed or not.
-                icon: 'link',
+                icon: 'fas-link',
                 label: 'core.openmodinbrowser',
                 action: (event: Event, module: CoreCourseModule, courseId: number): void => {
                     openUrl(module, courseId);