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/utils.py

@@ -16,7 +16,7 @@ import pyexcel
 import pyexcel_xlsx
 import pytz
 from dateutil import parser
-from flask import abort, current_app, redirect, request, session, url_for
+from flask import abort, current_app, g, redirect, request, session, url_for
 from flask_login import current_user, login_required
 from notifications_utils.field import Field
 from notifications_utils.formatters import (
@@ -767,3 +767,11 @@ def is_less_than_90_days_ago(date_from_db):
     return (datetime.utcnow() - datetime.strptime(
         date_from_db, "%Y-%m-%dT%H:%M:%S.%fZ"
     )).days < 90
+
+
+def hide_from_search_engines(f):
+    @wraps(f)
+    def decorated_function(*args, **kwargs):
+        g.hide_from_search_engines = True
+        return f(*args, **kwargs)
+    return decorated_function