MDL-30760 question engine: reduce summary max size

MySQL only accepts 65536 (aprox) bytes in default TEXT
columns. So we define the max allowed as 32000 to allow
99.9% of utf-8 contents to fit. If some day MDL-19603 is
implemented and all current TEXTs are moved (MySQL) to
the BIG counterparts, this restriction can be out (MDL-19603).

question/engine/bank.php

@@ -42,7 +42,8 @@
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 abstract class question_bank {
-    const MAX_SUMMARY_LENGTH = 65000;
+    // TODO: This limit can be deleted if someday we move all TEXTS to BIG ones. MDL-19603
+    const MAX_SUMMARY_LENGTH = 32000;
 
     /** @var array question type name => question_type subclass. */
     private static $questiontypes = array();