Remove DataImportMode.Elements

Part of #657 (closed)

This also causes the import task to be renamed to initialisation for Workers processes, as it was done before for Elements.

mentioned in merge request frontend!837 (merged)

mentioned in merge request tasks!200 (merged)

requested review from @babadie

changed the description

added 3 commits

• 1302f9d5...a088a46d - 2 commits from branch master
• 189faa63 - Remove DataImportMode.Elements

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merged

mentioned in commit a7d8581a