Add notion of local definitons in ZF

See merge request !220

- Fix syntax, code style, comment alignment
- Changed cleanjs_start from macro (???) to rhs
- Move example of let in ZF to bs_zf instead of bs_let
- Revert change to syntax_code in bs_let

Change cloogle's unofficiality status to official

See merge request !218

Add banners for AP 18/19

See merge request !219

Banner for the exam, the resit and the practice session

Update Platform & Cloogle for performance improvements; remove ABC interpreter settings from project files

This will cause a longer timeout when updating, but the fork apparently
causes performances issues

Update backend/build.sh; in util/update.sh don't exit when fetching artifacts from backend does not succeed (due to set -e)

Temporarily use cloogle branch of clean-platform which does not suffer from clean-compiler-and-rts/compiler#4

Update Syntax.icl

See merge request !217

In clean-compiler-and-rts/6b890e35e78c6cde75947593efd4b7776caf5b0b the restriction was lifted. It appeared that it forgot to copy it to the icl