Set default rate limits
Per Krinkle's recommendation in T133109#2434890.
For new users and IPs, it's 10 new short codes in 2 minutes.
For normal users, it's 50 new short codes in 2 minutes.
This only applies to when creating a new short code, so it shouldn't
affect most users who will typically visit articles that already have
been shortened.
Bug: T133109
Change-Id: I60a34c74ec037857d2a2782796a2293735df815e