Since jQuery 1.6 the attr() and prop() functions more strictly follow
W3C forms specification. Concretely the following is the current
functionality without going to deep into details:

* attr('checked') will actually check defaultChecked, which is the
  initial state of the checkbox upon page load.
* prop('checked') matches the current state of the checkbox. It will
  change state when the user checks or unchecks the checkbox.

https://api.jquery.com/attr/
https://api.jquery.com/prop/

Concretely current implementation is broken since probably jQuery 1.6 as
observed in devtools requests. The requests when toggling checkboxes
are always sending the on-page-load current value. So if a setting is on
all toggles of the checkbox will always turn the setting on server-side,
making it impossible to actually change the setting.

Switch to prop() function to resolve the issue.

Fixes #121
Fixes #166
Fixes #305

Signed-off-by: Melvin Vermeeren <[email protected]>