Add TagName linter

Add a linter `TagName` that ensures tag names are always lowercase.

Change-Id: I77ec1a9eb60cdadf4ee20f437772dd279158b599
Reviewed-on: https://gerrit.causes.com/28632
Tested-by: Shane da Silva <shane@causes.com>
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>