Request | Add option to change DateTime format in Web & Mobile apps

:bulb: **Idea**
Add an option to change the datetime format in the web and mobile apps.

Makes sense to have the default match locale, but having an option to manually choose would be awesome. For example I'm from Australia, but I much prefer to use ISO8601 date format (`YYYY-MM-DD`).

:computer: **Target components**

- Ntfy Web App
- Android, iOS
