Webhooks improvements

[anatskiy]

This PR improves and simplifies the overall Webhooks' structure.

1. All the webhook's files now are located in one folder (no more config, helper and static subfolders).
2. Function Webhooks.load() is unified even more for all endpoints (e.g., onload, masthead, tool, etc.).
3. Webhooks have a weight now (like it was explained here).
4. Some of the logic has been moved to the client (like it was suggested here).

If you agree to the changes, I'll update the documentation accordingly.

P.S. Every single webhook is broken now. I've updated all the demo webhooks in this PR, but others won't work.

[martenson]

@anatskiy This needs some de-conflicting, will you find time to do that?

[dannon]

(superseded by the rebased branch at #5342, testing/discussion/merging will continue there)