CI: run Credo in strict mode

Exclude test files from AliasUsage check in .credo.exs.
Use mix credo --strict in Justfile and .drone.yml.

Justfile

@@ -31,7 +31,7 @@ gettext:
 lint:
     mix format --check-formatted
     mix compile --warnings-as-errors
-    mix credo
+    mix credo --strict
     # Check that all German translations are filled (UI must be in German)
     @bash -c 'for file in priv/gettext/de/LC_MESSAGES/*.po; do awk "/^msgid \"\"$/{header=1; next} /^msgid /{header=0} /^msgstr \"\"$/ && !header{print FILENAME\":\"NR\": \" \$0; exit 1}" "$file" || exit 1; done'
     mix gettext.extract --check-up-to-date