The login button should show SpreadsheetAccess when on a spreadsheet election

[elboletaire]

I'm not sure if it's really a good idea to change the default login button behavior... it'll be better to create a new login button that unifies this logic, because I can see cases where developers would like to maintain the normal login button.

In implementations like ui-scaffold, where there's always the login button visible, in spreadsheet census strategy elections can lead to confusion, since they use their custom login flow.

Let's create a new login button that would show the spreadsheet login modal on these cases. This may require creating a new context for the modal, but it's worth studying it.