Note: the submitted date format is iso8601 YYYY-MM-DD (as we expect),
even if the displayed date by the browser depends of the user's browser
locale. Theses browsers then safely ignore input placeholder.
This is why we can't display the expected date format elsewhere in the
interface: these users will see another format as the one we expect, and
we don't want to mislead them.
The date format is only displayed as a placeholder, for browser
non-compatible with input of type date.
Validation is made when leaving the input field with an ajax call on the
/checks/support.json path. JSON response include the normalized domain
name (more TODO) and the supported status.
UI is updated with Javascript accordingly to this response.
Closes#62
