Use built-in "validateSchema" instead of custom implementation

[artem-zakharchenko]

Currently JSON Schema validator class implements its own validateSchema method using AJV under the hood:

gavel.js/lib/validators/json-schema-next.js

Lines 104 to 121 in 5884b69

	/**
	* Validates the schema against its version specification.
	* @return {boolean}
	*/
	validateSchema() {
	const { jsonSchemaVersion, jsonSchema } = this;
	const ajv = new Ajv();
	const metaSchema = META_SCHEMA[jsonSchemaVersion];
	ajv.addMetaSchema(metaSchema, 'meta');
	const isSchemaValid = ajv.validateSchema(jsonSchema);
	// Clean up the added meta schema
	ajv.removeSchema('meta');
	return isSchemaValid;
	}

With the same method implemented on the JsonSchemaLegacy class using TV4:

gavel.js/lib/validators/json-schema-legacy.js

Lines 59 to 72 in 5884b69

	validateSchema() {
	const { jsonSchema, jsonMetaSchema } = this;
	// In case schema version is unidentified,
	// assume JSON Schema Draft V3.
	const metaSchema = jsonMetaSchema || META_SCHEMA.draftV3;
	tv4.reset();
	tv4.addSchema('', metaSchema);
	tv4.addSchema(metaSchema.$schema, metaSchema);
	const validationResult = tv4.validateResult(jsonSchema, metaSchema);
	return validationResult.valid;
	}

I believe it's retained due to historical reasons and should be replaced with the build-in validateSchema option of AJV:

gavel.js/lib/validators/json-schema-next.js

Lines 147 to 155 in 5884b69

	const ajv = new Ajv({
	// Disable adding JSON Schema Draft 7 meta schema by default.
	// Allows to always add a meta schema depending on the schema version.
	meta: false,
	// No need to validate schema again, already validated
	// in "validateSchema()" method.
	validateSchema: false,
	jsonPointers: true
	});