Time picker doesn't respect military time formatting

[jikamens]

One of my users reported:

I use military time on my computer [0900 or 2100, for example] but when I use your extension it uses AM and PM – I remember your earlier extension adapted to me [for example I like to write emails between 0700 and 2200 but the rules show 7:00 AM and 10:00 PM] – why is that? I know it’s just a little quirk and not critical but it is one of those things I often wonder about. The screenshot regarding what I am talking about is included.

Thunderbird recently majorly revamped how localization of date and time formatting works, so it's possible that the configuration the user has in their OS for 24-hour time display was visible to Thunderbird before but isn't any longer.

I don't have time to look into this further right now so I'm filing this bug as a placeholder so that I can at some point come back to it.