Switch from yarn to npm

No other Wagtail project uses yarn.

.github/workflows/CI.yml

@@ -17,8 +17,8 @@ jobs:
       - uses: actions/setup-node@v2-beta
         with:
           node-version: "${{ steps.nvm.outputs.NVMRC }}"
-      - run: yarn install
-      - run: yarn frontend
+      - run: npm install
+      - run: npm run frontend
       - uses: actions/setup-python@v2
         with:
           python-version: 3.7

Makefile

@@ -112,7 +112,7 @@ docs: ## Regenerate Sphinx HTML documentation, including API docs
 
 .PHONY: frontend
 frontend: ## Compile frontend files
-	yarn run frontend
+	npm run frontend
 
 
 .PHONY: install
@@ -167,7 +167,7 @@ setup: setup-python setup-frontend ## Setup all
 
 .PHONY: setup-frontend
 setup-frontend: ##- Setup node modules
-	yarn install
+	npm install
 
 
 .PHONY: setup-python

docs/development.rst

@@ -6,7 +6,7 @@
 Development
 ===========
 
-For local development you need a system with Node v12.x, Yarn, Python3, Git and
+For local development you need a system with Node v12.x, NPM, Python3, Git and
 make. It is strongly recommended to use a Python virtual environment (`venv`_).
 The build process derives the version from repository data, so it's necessary
 to clone the repository and not just download a single snapshot.
@@ -83,16 +83,7 @@ check the rendering.
 Javascript package management
 =============================
 
-Use `yarn` for package management.
-
-To upgrade a Javascript package to a newer version use the following. This
-keeps `package.json` and `yarn.lock` in sync as BOTH will be modified:
-
-.. code-block:: shell
-
-   # ATTENTION: Select only those with reasonable version numbers.
-   # Be very careful if the MAJOR number would change
-   yarn upgrade-interactive --latest
+Use `npm` for package management.
 
 
 Release process
@@ -112,8 +103,8 @@ With an active virtual environment:
     python -m pip install --upgrade -r requirements.txt
     make clean
     make clean-frontend
-    yarn
-    yarn build
+    npm
+    npm run build
     prerelease
     python -m build
     python -m twine upload --repository pypi dist/*

netlify.toml

@@ -2,7 +2,7 @@
 # unless otherwise overridden by more specific contexts.
 [build]
   publish = "docs/_build/html"
-  command = "yarn build && pip install -e . && make docs"
+  command = "npm build && pip install -e . && make docs"
 
 [build.environment]
   PYTHON_VERSION = "3.7"