Validate against empty form submission for find_users_by_email

This included:
- creating a new form SearchUsersByEmailForm with validation
on its search field

- introducing 400 status to the view  if the form does not validate

- fixing the POST request data structure in the tests (it was
incorrect before and uncaught due to lack of validation and mocking
the response from the API.

app/main/views/find_users.py

@@ -4,19 +4,22 @@ from flask_login import login_required
 from app import user_api_client
 from app.main import main
 from app.utils import user_is_platform_admin
-from app.main.forms import SearchUsersForm
+from app.main.forms import SearchUsersByEmailForm
 
 
 @main.route("/find-users-by-email", methods=['GET', 'POST'])
 @login_required
 @user_is_platform_admin
 def find_users_by_email():
-    form = SearchUsersForm()
+    form = SearchUsersByEmailForm()
     users_found = None
+    status = 200
     if form.validate_on_submit():
         users_found = user_api_client.find_users_by_full_or_partial_email(form.search.data)['data']
+    elif request.method == 'POST':
+        status = 400
     return render_template(
         'views/find-users/find-users-by-email.html',
         form=form,
         users_found=users_found
-    )
+    ), status