replace strtolower calls with Common::mb_strtolower to prevent twig error " The string to escape is not a valid UTF-8 string"

Goal:
- Replace most calls to `strtolower` by `Common::mb_strtolower`
- Similarly replace calls to `strtoupper`, `ucfirst`, and other non unicode safe functions (maybe this could be done in a separate issue if this one focuses on `strtolower`)

Follows up https://github.com/piwik/piwik/pull/10050 https://github.com/piwik/piwik/issues/4410
