utf8mb4 and database creation (former PR #6527)

New PR needed as cannot bring old commit comments in line with new requirements

Changes brought into this new branch
- database creation option and choice of utf8/utf8mb4 for tables
- missing exception:title key; fixed schema issue with full text key; improved user creation logic
- function checkDatabaseSettings now uses version_compare for MySQL version check
- function checkDatabaseSettings now adopts early-return style
- in install/en.php file, improved introductory text for install step 3 (database)

Previous discussion on this feature at https://github.com/Elgg/Elgg/pull/6527