Prevent UI popping during subscription process

[begedin]

What's in this PR?

Closes #811

This makes it so while anything is happening in the background on the donate page, both the card form and the card item display are replaced by a "Processing..." indicator, which includes a loading spinner.

Since the change eliminates the need for some logic in the credit-card component (namely, the busy state), commit 260044f also removes all of that code.

If this behavior is not an acceptable solution, the only alternative I can think of is to show the form until the whole process is done, provided the form was there at the start, or show the card item until the process is done, provided the card item was there to begin with.

The problem with that is, it would significantly complicate the logic. In order to achieve that, we should

• revert 260044f
• remove the isBusy wrapping from the template
• on templates/project/donate, replace {{#donation/donation-container card=user.stripePlatformCardwith just card=card
• in routes/project/donate, add a model hook that explicitly loads the project and the user's card. Then add a setupController hook that sets the two properties on the controller. That way, the card wont be set automatically and the UI wont switch from the form to the card half-way through.

If the current solution is acceptable, finalizing this PR is a matter of setting the right style (in case the current spinner with "Processing..." is not good).

If the proposed alternative is the way to go, then the process id described above. I don't think it would break any tests to do it that way, but as explained, it slightly complicates things.

[begedin]

This was actually useless property we don't need anymore, so I would keep this one change in no matter what.

[joshsmith]

We can fix this by simply doing:

{{#if (and card (not isBusy))}}

Will need to install this https://github.com/jmurphyau/ember-truth-helpers