Remove usage of decode_entities()

I have done a reroll and i haven't found the function, only the declaration in file core/includes/unicode.inc

Here's my attempt at a reroll. A number of functions like hook_theme() were moved from core/modules/system/system.api.php to core/modules/system/theme.api.php causing a conflict in system.api.php. This has been resolved.

You're right in #3 and #7, we were dealing with removing drupal_mail as well, which is outside the scope of this issue.

I double checked the core codebase and can confirm you updated every use of decode_entities() in patch #8 outside of the function definition itself in core/includes/unicode.inc. Looks good to me.

Can some one add this issue to the correct change record so we have traceability - once done can be set back to rtbc.

Referenced this issue on the following change record: New Unicode component

This issue is a prioritized change as per #2350615: [policy, no patch] What changes can be accepted during the Drupal 8 beta phase? and it's benefits outweigh any disruption. Committed 32913b0 and pushed to 8.0.x. Thanks!