This column often breaks remaps because of some weird, long URLs. The data isn't that important, so truncating a couple of URLs doesn't hurt that much.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Cuts number of queries from 273 to 89
* Add some specs
* For a table with 500 posts, benchmarks locally shows a runtime
reduction from 0.046929135 to 0.032694705.
- new hidden site setting 'migrate_to_new_scheme' (defaults to false)
- new rake tasks to toggle migration to new scheme
- FIX: migrate_to_new_scheme also works with CDN
- PERF: improve perf of the DbHelper.remap method
- REFACTOR: UrlHelper is now a class