feat(scripts): add an ignore file for the explain script

Some of our existing stored procedures fail in the explain script but we
don't want those failures to show up. In the future we may have reasons
for wanting to ignore other procedures too. This change lets us ignore
them while still running the script in CI.

.explain-ignore

@@ -0,0 +1,4 @@
+accountEmails_4
+fetchVerificationReminders_2
+prune_7
+sessionWithDevice_15

scripts/explain-warn.js

@@ -68,7 +68,9 @@ Mysql(log, require('../db-server').errors)
   .then(db => {
     return populateDatabase(db, 0)
       .then(() => {
+        const ignore = parseIgnoreFile()
         return getProcedureNames()
+          .filter(procedure => ! ignore.has(procedure))
           .map(procedure => ({
             procedure,
             path: getPath(procedure)
@@ -272,6 +274,15 @@ function createAccountResetToken (db, uid, tokenId) {
     }))
 }
 
+function parseIgnoreFile () {
+  return new Set(
+    fs.readFileSync('.explain-ignore', RETURN_STRING)
+      .split('\n')
+      .map(procedure => procedure.trim())
+      .filter(procedure => !! procedure)
+  )
+}
+
 function getProcedureNames () {
   return cp.execSync(
     // eslint-disable-next-line quotes