✨(models/api) allow inviting external users to a document by their email

We want to be able to share a document with a person even if this person
does not have an account in impress yet.

This code is ported from https://github.com/numerique-gouv/people.

.github/workflows/impress.yml

@@ -71,6 +71,11 @@ jobs:
         run: yarn install --frozen-lockfile
       - name: Build mails
         run: yarn build
+      - name: Persist mails' templates
+        uses: actions/upload-artifact@v4
+        with:
+            name: mails-templates
+            path: src/backend/core/templates/mail
 
   lint-back:
     runs-on: ubuntu-latest
@@ -96,6 +101,7 @@ jobs:
 
   test-back:
     runs-on: ubuntu-latest
+    needs: build-mails
 
     defaults:
       run:
@@ -137,6 +143,12 @@ jobs:
           sudo mkdir -p /data/media && \
           sudo mkdir -p /data/static
 
+      - name: Download mails' templates
+        uses: actions/download-artifact@v4
+        with:
+          name: mails-templates
+          path: src/backend/core/templates/mail
+
       - name: Start Minio
         run: |
           docker pull minio/minio