Ensure focus indicator works in Firefox

fixes ADMIN-2675

Test plan
- Ensure focus styles work and look nice
  in all supported browsers

Change-Id: I54f1d46e422322bad1adc9d240e9f565465f5654
Reviewed-on: https://gerrit.instructure.com/196728
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Daniel Sasaki <dsasaki@instructure.com>

app/stylesheets/components/_imageSearch.scss

@@ -43,6 +43,11 @@
   background: transparent;
   border: none;
   padding: 0;
+
+  &:focus {
+    outline: 2px solid var(--ic-brand-primary);
+    outline-offset: 2px;
+  }
 }
 
 .ImageSearch__screenreader {
@@ -87,4 +92,8 @@
     overflow: wrap;
     margin: 0 .25rem;
   }
+
+  & a:focus, & a:hover {
+    outline: 1px solid rgba($ic-color-light, 1);
+  }
 }