Update database instructions to work in .NET Core 3.0

In the 3.0 SDK `dotnet ef` needs to be installed. This updates the instructions/tool config to do so.

More info: https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dotnet

src/Web/.config/dotnet-tools.json

@@ -0,0 +1,12 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "dotnet-ef": {
+      "version": "3.0.0",
+      "commands": [
+        "dotnet-ef"
+      ]
+    }
+  }
+}