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Bitbucket hook

https://github.com/nijel/weblate/pull/208#discussion-diff-3009481

Less code duplication and the _hook_helper functions returning an
HttpResponseBadRequest() is not handled by git_service_hook()
either way.

This reverts commit dc3e25a1.

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Apparently the documentation for writing brokers[1] is
wrong/outdated, and the documentation for managing the POST Service
broker is more up to date. So updated the code accordingly.

[1] https://confluence.atlassian.com/display/BITBUCKET/Writing+Brokers+for+Bitbucket
[2] https://confluence.atlassian.com/display/BITBUCKET/POST+Service+Management

Mostly (slightly) reworked code from github/bitbucket_hook().

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