Don't use internal templates anymore.

Blag will throw an error if a template is not found locally. The error
message contains an explanation for the user on where to get the missing
templates.

Quickstart will generate templates for the user in the working
directory.

Updated tests appropriately.

CHANGELOG.md

@@ -1,5 +1,20 @@
 # Changelog
 
+## [unreleased]
+
+### Breaking
+
+* blag does not use default fallback templates anymore and will return an error
+  if it is unable to find required templates, e.g. in `templates/`.
+
+  Users upgrading from older versions can either run `blag quickstart` (don't
+  forget to backup your `config.ini` or copy the templates from blag's
+  resources (the resource path is shown in the error message).
+
+  New users are not affected as `blag quickstart` will generate the needed
+  templates.
+
+
 ## [1.5.0] - 2023-04-16
 
 * moved to pyproject.toml