Enhancement: Add link control to verse block toolbar

[nfmohit]

Description

This PR addresses #6696 which requested the link control in the "Verse" block toolbar as we can see in the other blocks.

How has this been tested?

This PR has been tested by adding a "Verse" block in Gutenberg editor and locating the link control of the block in the block toolbar. It was also made sure that the link was correctly saved and rendered in the front-end. This was tested in WP 4.9.5, Gutenberg 2.8.0, Apache server with PHP 7.2.0 and MySQL 5.6.34. According to initial tests, the code doesn’t seem to affect any other areas.

Screenshots

Types of changes

This PR just removes the formattingControls attribute from the RichText element which specified the block to just have the bold, italic and strikethrough controls.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[noisysocks]

This was removed in bd676b5 which makes me think that it's intentional that we don't support adding links to Verse. Is that right, @mtias?

[aduth]

Based on @noisysocks 's comment, I take it to be intentional that this block not support link.