Drop Prettier and just use ESLint

There have been problems for a while with Prettier and ESLint fighting
with each other (despite ESLint being configured to not do formatting).
ESLint can do everything we want in the way of both formatting and
linting. So I have removed Prettier and we are now just using ESLint

.eslintignore

@@ -0,0 +1 @@
+/migrations

.eslintrc.json

@@ -0,0 +1,52 @@
+{
+    "env": {
+        "node": true,
+        "commonjs": true,
+        "jest": true,
+        "es6": true
+    },
+    "extends": "eslint:recommended",
+    "parserOptions": {
+        "ecmaVersion": 12
+    },
+    "rules": {
+        "max-len": [
+            "error", {
+                "code": 80,
+                "ignoreStrings": true,
+                "ignoreTemplateLiterals": true
+            }
+        ],
+        "indent": [
+            "error",
+            4
+        ],
+        "linebreak-style": [
+            "error",
+            "unix"
+        ],
+        "quotes": [
+            "error",
+            "single"
+        ],
+        "semi": [
+            "error",
+            "always"
+        ],
+        "comma-dangle": [
+            "error",
+            "never"
+        ]
+    },
+    "overrides": [
+        {
+            "files": [ "*.yaml" ],
+            "rules": {
+                "indent": [
+                    "error",
+                    2
+                ]
+            }
+        }
+    ]
+}