Hide persistent warnings when contributing

**Check #113 before, please.**

This is a proposal to "This notification should not pop up during the checkout." proposed by @snconnolly on #93. Currently, we have [2 persistent warnings](https://github.com/neighborly/neighborly/blob/86cc63367981f25621a174c34186e19cb88d42cd/app/controllers/concerns/persistent_warnings.rb#L16-L17) in the system: one asking to confirm the account and other to complete profile. I just refactored how we flash messages, so we will never show more than one notice at the top of the layout, as already reported. By accepting #113, we define the priority in this way:

1. Message specific of each page.
2. Asking to confirm account.
3. Asking to complete profile.

If none is needed, the user won't see messages.

My idea is: we get rid of persistent warnings (number **2** and **3**) in the contribution process. From clicking on "Contribute" in a project's page, the user won't see them until finish the payment of the contribution.