refactor: move code_suggestions to common folder

This is the last MR from the series of refactoring to separate the codebase to browser-specific, desktop-specific and common code.

Code suggestions are actively developed and so I was leaving them for last to minimize merge conflicts.

The goal of this MR is to move the code_suggestions folder (feature) to the common folder and remove all dependencies on the desktop folder.

This MR is best reviewed commit-by-commit.

Related to gitlab#412788 (closed)