Optimize total count of feed when loading activities in user dashboard. (#33841)

Two SQLs are very slow when `action` table have over 5M records.

```
database duration=1.8881s db.sql="SELECT created_unix DIV 900 * 900 AS timestamp, count(user_id) as contributions FROM `action` WHERE user_id=? AND act_user_id=? AND (created_unix > ?) GROUP BY timestamp ORDER BY timestamp"

database duration=1.5408s db.sql="SELECT count(*) FROM `action` WHERE (user_id = ?) AND (is_deleted = ?)"
```

This will cache the count for the first loading or when the activities
changed.

models/activities/action.go

@@ -445,6 +445,7 @@ type GetFeedsOptions struct {
 	OnlyPerformedBy bool                   // only actions performed by requested user
 	IncludeDeleted  bool                   // include deleted actions
 	Date            string                 // the day we want activity for: YYYY-MM-DD
+	DontCount       bool                   // do counting in GetFeeds
 }
 
 // ActivityReadable return whether doer can read activities of user