Update button URL to CDN image

[remcotolsma]

In Event Espresso we still link to an old iDEAL logo:
https://github.com/pronamic/wp-pronamic-pay/blob/develop/images/ideal/icon-64x48.png

We also no longer want to add full URL's to plugin files to the database:
http://localhost:8888/wp-content/plugins/wp-pronamic-ideal/images/ideal/icon-64x48.png

This makes it impossible to change or delete these files in an update.

For the URL settings we prefer an external CDN URL:

• https://www.jsdelivr.com/package/npm/@wp-pay/logos?path=dist%2Fmethods%2Fideal
• https://cdn.jsdelivr.net/npm/@wp-pay/[email protected]/dist/methods/ideal/method-ideal-640x360.svg
• https://github.com/pronamic/wp-pay-core/search?q=cdn

[remcotolsma]

I removed the default button URL completely in #6.