Use meta tag to tell search engines not to index

Google’s documentation says:

> robots.txt is not a mechanism for keeping a web page out of Google. To
> keep a web page out of Google, you should use noindex directives

A noindex directive means adding the following meta tag to pages that
shouldn’t be indexed:
```html
<meta name="robots" content="noindex" />
```

It’s also possible to set the directive as a HTTP header, but this seems
trickier to achieve on a per-view basis in Flask.

I’ve implemented this as a decorator so it can quickly be added to any
other pages that we decide shouldn’t appear in search results.

app/main/views/sign_in.py

@@ -14,9 +14,11 @@ from app import login_manager
 from app.main import main
 from app.main.forms import LoginForm
 from app.models.user import InvitedUser, User
+from app.utils import hide_from_search_engines
 
 
 @main.route('/sign-in', methods=(['GET', 'POST']))
+@hide_from_search_engines
 def sign_in():
     if current_user and current_user.is_authenticated:
         return redirect(url_for('main.show_accounts_or_dashboard'))