[FEATURE] Validation for TOTAL file upload size

[georgringer]

This is a followup of #456 (comment)

There is plugin.tx_powermail.settings.misc.uploadSize which is a limitation for a single file but what is missing is a limit for the sum of files.
Reasons are simple: there can be a limitation at the mail server with e.g. 10mb. Currently if the uploadSize is set to e.g. 3mb it is still possible to hit the limit with 4 files and it is also not possible anymore to just send one 9mb file