Implement Native PayButton Generator

[xecdev]

This PR implements issue #1 by adding native support for a PayButton generator. The new generator is a streamlined version of the Button Generator found on PayButton.org. With this enhancement, WordPress administrators can easily create simple buttons (for donations, "buy me a coffee", etc.) and embed them into posts or pages using a shortcode.

Test plan:

• Install the plugin
• Go to the Button Generator page (either from the menu or from the PayButton Dashboard)
• If the styling of the Button Generator page seems off, then clear the cache by hard-refreshing the page (WP caches CSS for admin pages)
• Generate a button
• Check the form's functionalities (Tooltips, dropdowns...)
• Copy-paste the generated shortcode to a post or page and verify that it works

[Klakurka]

• When attempting to update the previously-installed plugin, it doesn't seem to do it and instead just installs the new one along side it so that it shows a Warning: Constant PAYBUTTON_PLUGIN_DIR already defined in C:\Users\david\Local Sites\paybutton\app\public\wp-content\plugins\wordpress-plugin-fix-improve-unlocked-content-indicator-style\paybutton.php on line 24 at the top of the Paywall Settings page and every page appears twice for me. Removing both and installing again fixed it.
• When switching to a BCH address, the primary colour should change to #4BC846. When going from BCH -> XEC, it should also change back the primary colour to 4BC846.
• Would be nice to have a copy button next to the 'shortcode'.

[xecdev]

Fixed. Please review it again.

[Klakurka]

• Click-to-copy doesn't work for me. It should both copy to clipboard and also display a 'Copied!' text instead of 'Click to copy!' when clicked.
• Editing the primary colour no longer works.