When a user accepts an invite with an email address that matches a group
which automatically awards its members Trust Level 2, a race condition might happen
when the 'send_advanced_tutorial_message' job runs.
That job is enqueued inside the 'user_promoted' event which is triggered inside a
transaction on the user record. If the job runs before the transaction is done,
the user record is invisible and this generates an exception.
FileHelper.download requires a string not a URI. I also found another
instance of using open-uri directly and swapped it out to use
FileHelper.
I also updated it to not `read` a file if it comes back nil.
Follow up to: fe01099a38
By requiring open-uri this will fix the following error:
```
NoMethodError (private method `open' called for #<URI::Generic...
```
also switched to the shorter syntax and removed default options. Since
ruby 2.4 redirect is on by default.
To eliminate a DDOS attack vector, we're taking the following measures:
The endpoint will be rate-limited to 3 requests every 60 seconds (per user).
A 24 hours max-age cache header is sent with the response.
The route will be hijacked to generate the certificate in the background.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging