Allow users to upload transactions for existing activities via a standalone template

[emmajclegg]

This follows work on XLS templates in issues #1439 and #1575 (which was paused) and an ODS assessment of current bulk import functionality.

The most popular user story we hear, related to bulk import, is:

“I want to upload multiple transactions for an activity in one go, to avoid me having to create them individually in the interface. I may want to upload transactions for multiple activities at once.”

At the moment, this involves the user exporting all of their activity data, adding transactions, then reimporting. Can we discuss first steps to making this workflow more intuitive to users?

We could:

• use aspects of this revised workflow, particularly the "importing elements from within activity" flow
• use a simplified version of the updated XLSX template: Transaction Template XLSX (simplified)
• minimise changes to existing export and import templates (for now)

We should avoid workflows:

• that require the user to download data, add to it and reimport (as this encourages the user to manage their data outside of IATI Publisher, rather than in the interface). Therefore the focus of this issue should be on adding new transactions to existing activities, not overwriting existing transactions.
• that allow activities to be created in a workbook and uploaded. IATI Publisher users should create activities in the interface.