Implement a check_database_schema performance check

1) Run the existing cli check: $ php admin/cli/check_database_schema.php Confirm it is OK 2) Run the new GUI version of this check: /report/performance/index.php Confirm it is ok 3) Look at the details of this report: /report/performance/index.php?detail=core_dbschema Confirm it is ok 4) Change the schema, eg in a db shell run: DROP INDEX mdl_extetoke_tok_ix; 5) Run the existing cli check: $ php admin/cli/check_database_schema.php Confirm it shows the error: external_tokens Missing index 'token' (not unique (token)). CREATE INDEX mdl_extetoke_tok_ix ON mdl_external_tokens (token); 6) Run the new GUI version of this check: /report/performance/index.php Confirm it the error status and summary 7) Click on 'Database schema check' to see the details of this report, or nav to: /report/performance/index.php?detail=core_dbschema 8) Confirm it shows the error details like the cli external_tokens Missing index 'token' (not unique (token)). CREATE INDEX mdl_extetoke_tok_ix ON mdl_external_tokens (token);