Switch back extensions.webextensions.ExtensionStorageIDB.enabled = true

In legacy/trac#31396 (moved) we set extensions.webextensions.ExtensionStorageIDB.enabled = false to avoid problems with the new idb storage for extensions and the Services.qms.clear() that we were doing in torbutton. After legacy/trac#30504 (moved) and legacy/trac#19417 (moved) were fixed, turning that pref back to the default should be possible.

However, we should check that it's not possible for browsers that were affected by the original bug (Services.qms.clear() clearing the internal indexeddbs) to end up in a bad state that the browser cannot recover from. We should make sure that the extensions.webextensions.ExtensionStorageIDB.migrated pref branch is cleared so that the migrations are performed on first start after a browser upgrade containing the fix.