Integration resync webhook fails

[humitos]

Steps to reproduce it:

1. Import GitLab project (do not use Manual Import)
2. Webhook are created sucessfully
3. Go to your GitLab project and remove the RTD integration from settings
4. Go to your Admin -> Integrations
5. Select your integration
6. Click on Resync

Message received:

Webhook activation failed. Make sure you have the necessary permissions.

The logs show:

[10/Apr/2018 12:52:17] readthedocs.oauth.services.gitlab:347[30538]: DEBUG GitLab webhook update failure response: {'message': '404 Not found'}

This is happening because we are trying to update a webhook in GitLab that it was manually removed by the user. So, we need to handle this case here and re-create the webhook if it fails when trying to update.

NOTE: you need ngrok setup to test this in your local instance

Related: #3587 #2561

[humitos]

The same happen in GitHub:

[10/Apr/2018 13:04:30] readthedocs.oauth.services.github:281[30538]: DEBUG GitHub webhook creation failure response: {'message': 'Not Found', 'documentation_url': 'https://developer.github.com/v3/repos/hooks/#edit-a-hook'}