E-Mails ignore locale setting

[simonspa]

Currently, the notification e-mails sent for activities ignore the locale set by the user. Looking at the code in https://github.com/nextcloud/activity/blob/master/lib/MailQueueHandler.php#L352 the mail handler seems to only read the language and time zone of the user but not the locale for date-time formatting.

Steps to reproduce

1. Set language to "English"
2. Set locale to "Germany"
3. Expect an e-mail in English language with German date/time formatting (24h)

Expected behaviour

Receive an e-mail in English language with proper 24h time format from "Germany" locale

Actual behaviour

Receive an e-mail in English language with 12h time format (AM/PM)

[joshtrichards]

Related: #424, #426