api: Improve feedback on unsaved changes

[peterthomassen]

@nils-wisiol wrote in #478:

I find it unintuitive that in the webapp token table, when I click the "Can manage tokens" slider, I still have to save that row. From my experience with Android, I'd assume it saves automatically/instantly. The background color change didn't help much in my case, as I only had two rows: the login token, dark grey becase disabled, and the current row. The current row was white to begin with, then light grey on hover, then later yellow because of unsaved changes - which I didn't notice. Two ways to improve this could be (in order of my preference): 1. introduce an indicator of unsaved changes on top of the table where the blue bar is currently (preferably in a way that doesn't move the page when it appears/disappears). 2. hide the save button of each row unless there are changes to save 3. auto-save the changes on the slider 4. popup-warning when you leave the page with unsaved changes