Revert "Revert "Added ability to match Variation in language tag eg en-GB+VARIATIONTAG""

This reverts commit 95ba8c5b4267310f8a153416016b8dc2847c083a.
Revert "Revert "- Changed wording around LanguageTag to support the official PrivateUse subtag""

This reverts commit 0b75d04f39823de4dcec84f87fdd7e17153062e4.
Revert "Revert "Added console error if PO generator finds blank lines in PO file""

This reverts commit c50680a9aa6adad3166cbf4dc4b9033d1e7f7288.
Added Private Use subtag for multitenant etc. scenarios

Will accept a tag with a private use subtag e.g. en-GB-x-Scot or
zh-Hant-HK-x-Tenant123
Adding references to new private use subtag
Added info to README.md about private use subtag
Editing comments
Editing comments
Merge remote-tracking branch 'origin/multitenant' into ExcludeUrlsFromProcessing

Conflicts:
	src/i18n/LocalizedApplication.cs