Import activities from Trello and Wekan #1187
                
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This PR is related to #1157
Now, when importing boards from Trello or Wekan, we import the original board activities in order to have the right dates of the activities. Except, there are two issues :
The cause of both issues are as follows, we can't - for now - circumvent hooks of CollectionFS, I opened an issue in the corresponding repository and once it is resolved, fixes will be implemented and we'll never have the first issue. As of the second issue, Trello doesn't export comments with their ids which prevented me from mapping between Trello ids and Wekan ids, I'll be looking for a solution for that.
This change is