maxfilesize on import should respect 0 as meaning "no limit"

UX improvement

At civicrm/admin/setting/misc, setting maxFileSize to 0 or leaving as an empty value should be interpreted as "no limit", per default behaviour of PHP ini_set for similar restrictions.

In places like CRM_Import_Form_DataSource::buildQuickForm(), we treat it as a literal value, so 0 will mean that files of any size cannot be uploaded.

Fix would be that if $config->maxFileSize is 0 or '' or otherwise false-y, do not add the rule for max file size in that form. (This probably needs to be applied elsewhere.)