Changed MySQL column type for redirect_url to "TEXT" to avoid truncation.

Fixes #1346

db/db_mysql/migrations/20190219214543_0.8.0_redirect_url_field.sql

@@ -0,0 +1,8 @@
+
+-- +goose Up
+-- SQL in section 'Up' is executed when this migration is applied
+ALTER TABLE `pages` MODIFY redirect_url TEXT;
+
+-- +goose Down
+-- SQL section 'Down' is executed when this migration is rolled back
+ALTER TABLE `pages` MODIFY redirect_url VARCHAR(255);