Fix the memory leak related to PDF generation for emails. This is for the migration of SaaS to OSD.

See https://github.com/3scale/porta/commit/dce8bb9f6f586b9dc76cd05f4551baa6cb4f2913
And maybe https://github.com/3scale/porta/commit/59359bd54424250b6a13c8f3fbf04cf9930bc82e
And maybe https://github.com/3scale/porta/commit/48ead266bfd5b284f69233f7b4b95163ba00a8aa
https://github.com/3scale/porta/compare/memory-leak/pdf-report

This is not finished, but was started by Hery. See above links.
He tried to use other gems to replace an outdated gem for PDF formatting. Those other gems work differently and our approach to generate PDFs from HTML snippets doesn't properly format them.

It may be better to try using just the lower level library instead of generating HTML snippets. It could be easier to achieve and maintain.