Optimize css flow

[bttf]

Users were experiencing issues editing the Global CSS of a page. This was primarily due to two reasons:

• the CSS saving pipeline was unoptimized (save requests made when no changes were effected)
• we got rid of optimistic saving a while ago, and it has cause some minor degradation of quality in UX; global CSS showing the extreme case.

This PR addresses the above two issues. In addition, we remove the use of error codes and instead use actual error messages instead, which should be more helpful in support situations.