Add linting to gitlab

.gitlab-ci.yml

 stages:
   - test
+  - lint
   - release
 
 
@@ -29,6 +30,23 @@ lts_211:
   before_script:
     - pip install .['testing'] wagtail~=2.11 django~=3.1
 
+
+flake8:
+    stage: lint
+    image: python:3.8
+    before_script:
+        - pip install flake8
+    script:
+        - flake8 wagtailvideos/ tests/
+
+isort:
+    stage: lint
+    image: python:3.8
+    before_script:
+        - pip install isort -e .
+    script:
+        - isort --recursive --diff --check-only wagtailvideos/ tests/
+
 build:
     image: python:3.8
     stage: release