correct colors on member links in channel topics

Fixes laCour#173

Before: https://cl.ly/3I283N0p2x2f
After:  https://cl.ly/3N1k37071e36

Test Plan:
- Edit the topic of a channel to include a mention of a user
- Verify that the background and color of the mention match the rest of
  the theme

scss/modules/flexpane/_mentions.scss

@@ -17,6 +17,11 @@
   color: $base-link-color;
 }
 
+a.internal_member_link {
+  background: $color-shade-dark;
+  color: $base-font-color;
+}
+
 a.c-member_slug,
 .c-member_slug--link,
 .c-mrkdwn__subteam--link {