Query tagging events and posting multiple datasets

[taxe10]

• Add API call to GET the datasets + tags associated with a tagging event, something like: API_URL_PREFIX + '/events/{uid}/datasets
• Add API calls to GET tagging event according to it's UID and to QUERY tagging events according to tagger_id
• POST multiple datasets (120k images) with their corresponding labels. Currently posting 1 by 1 takes too long and there may be duplicated URIs
• Add unit tests for all these changes