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
Note, current practice if for plugins to submit PRs to core for any
migrations required for plugins, so we can better control schema.
Especially if core tables are being touched.
In this case index has close to zero cost unless akismet is installed
This reduces the akismet admin query from 20ms on every new page load to 0.5ms