BREAKING FEATURE: move user data

- Create ./data/users/ folder
- Move user configuration to ./data/users/username/config.php
- Move sqlite db to ./data/users/username/db.sqlite
- Move user logs to ./data/users/username/log.txt

See https://github.com/FreshRSS/FreshRSS/issues/729
Fix listUsers function

See https://github.com/FreshRSS/FreshRSS/issues/729
Fix getting db size for sqlite

See https://github.com/FreshRSS/FreshRSS/issues/729
Merge branch 'dev' into 729-move_data
Fix install.php to consider new data organization

- Introduce USERS_PATH
- Remove LOG_PATH
- Use join_path in install.php
- Change references to old  data files to the new ones
- Update i18n

See https://github.com/FreshRSS/FreshRSS/issues/729
Remove all old references to LOG_PATH

See https://github.com/FreshRSS/FreshRSS/issues/729
Add a default users/_ dir

Useful for default log path

See https://github.com/FreshRSS/FreshRSS/issues/729
Merge branch '729-move_data' into dev

To update, please follow instructions in https://github.com/FreshRSS/FreshRSS/issues/729