use exact match for username search in API admin

Changes that we want to make to username lookup don't allow wildcard
lookups, so revert to exact

Author: alastair

apiv2/admin.py

@@ -26,7 +26,7 @@
 @admin.register(ApiV2Client)
 class ApiV2ClientAdmin(admin.ModelAdmin):
     raw_id_fields = ("user",)
-    search_fields = ("user__username", "name", "=oauth_client__client_id", "=key", "description")
+    search_fields = ("=user__username", "name", "=oauth_client__client_id", "=key", "description")
     list_filter = ("status", "throttling_level")
     list_display = ("name", "url", "user", "status", "throttling_level")