Add signup form carousel on landing page
Initial payments infrastructure

WIP

The release of Square Cash provides a unique service that enables us to
integrate paying off debts entirely within the platform without actually
processing any transactions. The extent of integration is basically
adding a mailto link with fields pre-formatted; however, a compelling
design and platform integration drive the feature.

A payments button is provided on the dashboard to send a Square Cash
email; this button is also instrumented to send data to the Payback
backend so we can track and display payment data.

As of now, we have no way to confirm whether the user has properly
registered or payment was successful - it's a 'fire and forget' system.
This should work for the time being, although it would be possible to
add a contention / reversal mechanism if need be.

The feature announcement page is a work in progress.

Other additions include:

- The nav bar is refactored to use stack dropdowns on mobile
  due to the addition of the Payments tab.

- The pages directory is removed from app/assets/stylesheets. That was
  dumb.

- The static layout is removed, and the logic is compressed into
  application.html.erb.

- Tipsy tooltips are refactored - instead of manually calling .tipsy()
  on each selector, a declarative data-tipsy attribute is now used.
Add content with placeholder images to cash landing page

Additionally, the page is moved back from the payments controller
to the static controller.
Add dashboard cash announcement
Rename payments table image
Finishing cash landing page
First implementation of payments for net totals

The very first implementation of payments was centered around the
expense. However, it'd be a convenience to be able to pay a net total
owed at once in a single payment. This adds a 'pay all button' to the
user debts page, which will automatically configure a payment with the
proper amount.

The difficulty is in tracking the mapping back to the original expenses.
If we create a single payment record with the amount equal to the total
amount owed, we have no mapping back to the expenses that was for, and
have no way to display that a particular expense has already been paid
for.

An alternate approach would be to create a payment record for each
expense paid for. However, this could be confusing to the user as the
payment was a single transfer representing a contiguous sum, and still
doesn't solve the problem of figuring how whether or not a user has paid
back another user in full.

This commit implements the first approach (a single total payment) - the
mapping item is left as a TODO.
First attempt at notifications refactor

The new payments notification type can correspond to multiple expenses
("Jill sent $99 for 3 expenses"). This requires a refactor of the
notifications infrastructure as previously a notification was tied to a
single expense.

The mark off button on the payments notification is still a work in
progress - it will work exactly the same as the mark off request type,
but needs to target multiple expenses.
Fix scss lints
Fix pay button checkoff
Finish payment notification JS interactions

This commit implements a similar mark off button / slide up
functionality for the payments notification button, which can
potentially be associated with multiple payments. Included is an option
to toggle a list of expense titles that a payment is associated with.

One final TODO is how to deal with outstanding credits from a user when
paying off a net debt. It makes the most sense to automatically mark
them off, but that needs to be clear to the user. Things could
potentially be confusing while waiting for the user to acknowledge the
payment, but the envelope icon should address this.
Improve padding on outlined cash button
Change payment confirmation behavior

Previously, payment buttons would be replaced with a green checkmark
after sending payment. The behavior is now consistent using the envelope
icon + timestsamped tooltip.
Tidy up payment email bodies
Fix notification/dashboard markoff behavior
Disable multi-expense payments for launch