Don't flag "EXPLAIN" as a write

Summary: These queries are safe to run without a CSRF token, and we need them
for the query analyzer in DarkConsole.

Test Plan: "Analyze Query Plans" works again.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, epriestley, nh

Differential Revision: 895