Introduces an image to a payment button

* Introduce Jetpack_Simple_Payments class, load it

* Add doc about format to CPT

* Add more docs

* Add enabled / disabled status

* Add shortcode

* reg shortcode

* Add shortcode logic

* Fix func

* fix class

* rename to product

* Better content passing

* Pass proper JS

* Add comments

* Rename to "paypal-express-checkout"

* Add price

* Label change

* jshint fixes

* filter

* items

* Throw error if paypal not defined

* Info about script hosted with PP

* Get rid of static keyword

* Fix some lint issues

* proper header

* Adjust classes to WP coding standards

* Change header

* Change comment

* add payload for number of items

* Make ids work for paypal code

* repair init code

* Introduce ID for form

* Add endpoint urls

* Better domain

* Endpoint urls with blog_id

* Fix id

* Add version

* Change endpoint

* Make sandbox

* Add post meta syncing

* Sandbox management

* Review fixes

* remove console

Attempt to make test pass in JS

* Fir for missing product

* simple-payments: print message when the transaction ends.

* simple-payments: clean message placeholder

* simple-payments: implement styles to the block

* some change

* simple-payments: improve showing transaction messages

* Put the purchase message into HTML, hidden by default

* Toggle purchase messages with either success/error and message

* Use p tag instead of div with margin

Props @iamtakashi for the suggestion.

* Remove unneeded code

* Fix typo

* Fix paypal checkout.js script error

* Use .html instead of text to append html content

* Removing one error message: there's no error to show, uneeded

* Fix toggle logic of the purchase message

* fix linting issue

* update success copy

* simple-payments: small coding improvements in output_shortcode()

* simple-payments: do not use underscore in css class
https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/#selectors

* simple-payments: minor improvements defining endpoints

* simple-payments: use `<p>` tag for description and price
#7500 (comment)

* simple-payments: minor improvements

* simple-payments: add purchase-box container. improve paypal button styles.

* Simple Payments: update error message handling (#7503)

- display success message from backend ( D6514-code )
- display all error messages

* simple-pyaments: wrap all text into `<p>` elems

* simple-payments: move globals as PaypalExpressCheckout properties

* simple-payments: coding improvementes.

* simple-payments: minor code improvements

* rebase: squash -> "simple-payments: move globals as PaypalExpressCheckout properties"

* Remove wpautop for Simple Payments product content/description (#7507)

* Simple payments: image (#7508)

Introduces an image to a payment button

* Disable default sandboxing

* simple-payments: user camel-case convention

* reenable sandbox

* Add env handling

* Simple Payments: Style shortcode

* simple-payments: inject msgs in the right place

* simple-payments: show error in onAuthorize callback

* fix additional error messages

* Simple Payments: Modify message style

* fix empty join call

* Simple Payments: Reset iframe margin

* wrap default error message

* Implement review feedback

* Add min field

* SimplePayments: add formatted price meta data (#7521)

Output `spay_formatted_price` if present

* somple-payments: improve-error-handling

* simple-payments: check processErrorMessage() param

* Fixed lint errors.

* Simple Payments: return wpcom blog_id if code is running on wpcom (#7524)

* Simple Payments: add method to process success messages (#7528)

Fixes styling of success messages.