Allow template zip to contain folder in root of archive

[jakejackson1]

Description
We get a lot of support tickets about trouble installing templates due to incorrectly zipping them up. https://docs.gravitypdf.com/v6/developers/install-template-via-template-manager#common-problems

Let's allow zip folders to contain one directory in the root of the archive and then the template(s) inside that folder.

Step To Reproduce
Steps to reproduce the behavior:

1. Zip up a folder that contains PDF templates
2. Upload via the PDF Template Manager

Expected behavior
Currently this will fail to install due to in invalid archive. The new behaviour will allow this behaviour if the zip contains one directory with the templates then inside it. Note: if there are multiple directories in the root of the archive this will still be considered invalid.