Measure usage of Special:Search did you mean

Data recorded:

• How many results the user was shown for their query
• Whether the user was shown a did you mean result
• Whether the user clicked on the did you mean result

Statistics that should be computable from this:

• Percentage of queries where the user was shown a "Did you mean" result
• Percentage of queries where, having been shown a "Did you mean" result, the user clicked on that result

Bug: T105201
Change-Id: I28d0aa860b3fd5cf865b52c774fe082f00ce475a