Accounts with user ids containing forward slashes can't be loaded correctly

[TobiasFella]

Reported as https://bugs.kde.org/show_bug.cgi?id=491252

The user id is used as a key for QSettings, which uses forward slashes as separators. When listing the account, wrong groups are thus returned.

We need to somehow come up with a way of escaping forward slashes while not breaking existing logged-in accounts (at least those that do not contain forward slashes; for the other ones, logging in again is probably inevitable but also not problematic. I haven't been able to come up with a nice way to do that yet.

[KitsuneRal]

I was about to say that forward slashes in user ids are a deprecated legacy but no ;(
Why can't we "simply" percent-encode the forward slashes though?