Update largo_donate_button to use Pico class

[joshdarby]

Changes

This pull request makes the following changes:

• Recreates the largo_donate_button function to add the PicoPlan class to the button so that the Pico modal will pop up, detailed here: https://github.com/INN/umbrella-sfpublicpress/issues/66#issuecomment-646830618

Will look like this on prod:

Why

For #66

Testing/Questions

Features that this PR affects:

• Donate button in header and anything else using largo_donate_button

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [x] How come the Largo Donate widget doesn't use a filterable version of largo_donate_button?
• [ ] Note: I doubt anything will appear when the button is clicked on your local environment unless you have the Pico plugin set up. If you want to try setting it up, install the plugin and log in to Pico with the creds in 1Pass.

Steps to test this PR:

1. Make sure nothing is broken

[benlk]

I doubt anything will appear when the button is clicked on your local environment unless you have the Pico plugin set up. If you want to try setting it up, install the plugin and log in to Pico with the creds in 1Pass.

The Pico plugin is installed and active and logged in, but I'm getting CORS and 422 errors in the browser console, and the popup never opens.

Should this link keep the current href as a fallback option?

How come the Largo Donate widget doesn't use a filterable version of largo_donate_button?

Looks like it's because that widget allows setting arbitrary links in the widget backend. largo_donate_button() has no arguments allowing text or link to be set, but we could add that in a future version of Largo, with fallback params being the current database-fetched items. Is it worth creating a ticket for that?