Revert Install Session Start

Revert install session start

First revert did not completely remove addition and causes warning on
install

- Completely remove require startup/session.php
Merge pull request #1288 from mkly/revert_install_session_start

Revert Install Session Start
page forbidden tweak because area is wrong anyway
gettin ready for 5.6.2.1 awwwww JEAH
Fix BlockController::getUniqueIdentifier()

Fix `BlockController::getUniqueIdentifier()`

- Use `getBlockID()` no `getIdentifier()` as `getIdentifier()` will
  already append the BLOCK_ prefix resulting in BLOCK_BLOCK_