Pseudlocalization (>>marking<< strings marked for translation, instead of actually translating them), is supported since #810 by setting `localStorage.test_l10n = true` in the browser console and reloading the page.

> Note: the current lingui implementation needs `npm run gettext:extract; npm run gettext:compile` for any new strings before they appear as marked. We should try to remove that limitation during the i18next switch.

This PR makes it easier to enable/disable by setting a `?pseudolocalization=true/false` URL param, which gets used to set/unset `localStorage.test_l10n`.
(Thus, after using `?pseudolocalization=true` once, you will see it until you explicitly set `?pseudolocalization=false`, even when visiting URLs without the param.)