Make WATCHMAN_STORAGE_PATH relative to Django's default storage

[pchiquet]

Before this change, watchman may use an invalid path when the default storage does not use MEDIA_ROOT.

For example :

• MEDIA_ROOT is set to "/media"
• the default storage is Amazon S3 (or another custom storage)

then, watchman tries to write to "/media/watchman-xx-xx.txt", which fails with :

ValueError: the joined path is located outside of the base path component
  File "storages/backends/s3boto3.py", line 377, in _normalize_name
    return safe_join(self.location, name)
  File "storages/utils.py", line 79, in safe_join
    raise ValueError('the joined path is located outside of the base path'
SuspiciousOperation: Attempted access to '/media/django-watchman-730f4aa9-3d7d-4931-9993-5e13b422afa3.txt' denied.
  File "watchman/decorators.py", line 28, in wrapped
  File "watchman/checks.py", line 63, in _check_storage
  File "django/core/files/storage.py", line 51, in save
    name = self.get_available_name(name, max_length=max_length)
  File "storages/backends/s3boto3.py", line 582, in get_available_name
    return super(S3Boto3Storage, self).get_available_name(name, max_length)
  File "django/core/files/storage.py", line 75, in get_available_name
    while self.exists(name) or (max_length and len(name) > max_length):
  File "storages/backends/s3boto3.py", line 474, in exists
    name = self._normalize_name(self._clean_name(name))
  File "storages/backends/s3boto3.py", line 380, in _normalize_name
    name)

[coveralls]

Coverage remained the same at 90.975% when pulling bfa3583ee2a6d2b8c0eabffc3cbd96c9effe8f5e on pchiquet:fix-storage-path into ab95718efa9d71cd2611f14a0cf9685ad8dfda11 on mwarkentin:master.

[coveralls]

Coverage remained the same at 90.975% when pulling bfa3583ee2a6d2b8c0eabffc3cbd96c9effe8f5e on pchiquet:fix-storage-path into ab95718efa9d71cd2611f14a0cf9685ad8dfda11 on mwarkentin:master.

[stale[bot]]

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