[QA] Heads up: localStorage caching enabled on beta cluster

[Ori Livneh]

Hey folks,

Just a heads up that I enabled an experimental feature on the beta
cluster: ResourceLoader module caching using localStorage
(https://gerrit.wikimedia.org/r/#/c/86867/). The feature uses storage
capabilities that are available in newer browsers to cache JavaScript
and CSS assets. We think that the overall effect will be a diminution
of the amount of data users need to fetch from our servers to render a
page. Older browsers that do not implement localStorage should not be
impacted by the change.

The patch has gone through several rounds of testing, and I think it's
ready for the beta cluster. But it does modify the way a central
component of MediaWiki behaves, so major breakage is always possible.
I will keep an I on it, but please let me know if you notice a spike
in errors. Be especially alert to JavaScript code that seems stuck in
a historic state.

If the breakage is severe, the feature can be disabled by reverting
https://gerrit.wikimedia.org/r/#/c/92460/ (or by setting
$wgResourceLoaderStorageEnabled to false in CommonSettings-labs.php).

Thanks!
---
Ori Livneh
ori at wikimedia.org