[CI] Add webpack steps to tox commands

tox.ini

@@ -10,7 +10,9 @@ skip_install = true
 envdir = {toxworkdir}/globalenv
 commands_pre =
      poetry install -E ldap
-     poetry run aleksis-admin yarn install
+     poetry run yarnpkg install
+     poetry run aleksis-admin write_webpack_entrypoints
+     poetry run yarnpkg run webpack
      poetry run aleksis-admin collectstatic --no-input
 commands =
     poetry run pytest --cov=. {posargs} aleksis/