PCH Smart Linking: Fix issues with undo functionality and link offsets

[vaurdan]

Description

This PR fixes a few issues with the PCH Smart Linking:

• As reported in PCH Smart Linking: Undo doesn't work for All Blocks option #2313, the undo was not working properly to revert the applied links in the full content. This has been addressed and CTRL/cmd + z should remove all the applied links.
• FIxed an issue when applying multiple links with the same text and different offsets.
• Improved the success notice and Snackbar to show the actual number of added links, instead of the number of link suggestions. The notice is now dismissible as well.

Motivation and context

Improve the reliability of the Smart Linking feature and the user experience when working with this feature.

How has this been tested?

Tested locally, with a specific link suggestions payload that included three suggestions with the same text, but three different offsets.