Dates: make the format configurable

[bpierre]

Not as urgent and straightforward as #1406, but it could be a better solution in the longer term.

ISO-8601 is good to represent dates without ambiguity, but letting users choose their preferred format would be a better option.

It would require the following:

• Add a way to pass it through aragon.js (see the addition of guiStyle as an example).
• Add a “Display” section to the global preferences, with the date format.
• Use it to format the client dates.
• Use it to format the apps dates (this could be done as a second step).

We could also think of adding other information and passing it all at once, e.g. we could have a i18n object containing the preferred date and time formats and the currency. If we decide to localize the client and the apps one day, this is also where the preferred language would go.