Old webhooks gives 502

[humitos]

Running this command,

$ http POST https://readthedocs.org/github repository:='{"url": "", "ssh_url": ""}' ref=""

breaks the site and may produce tons of tasks being ran.

After running that command I get 502 and I tried it locally and I found that the problem is at the Project query since all the repo urls ends with ''

https://github.com/rtfd/readthedocs.org/blob/445683fc0c86b0de56779447a8eedf98024dba32/readthedocs/core/views/hooks.py#L92-L96

Finally, update_imported_docs is triggered by each of the project.

The only line that could save this execution is

https://github.com/rtfd/readthedocs.org/blob/445683fc0c86b0de56779447a8eedf98024dba32/readthedocs/core/views/hooks.py#L120

Related to #3520

[stsewd]

Should this be fixed on the query or when parsing the request data?