Wait for transactions to actually complete in IndexedDB write operations.

[LTLA]

• All onerror statements will now automatically reject with an appropriate error message.
• Removed helpers and just repeated the code in the relevant function bodies. This is desirable as IndexedDB operations are closely tied to the event loop and it helps to see that everything is happening in the expected sequence.

[LTLA]

@jkanche please try it out (recommend flushing your IndexedDB stores to check that the downloads go well).

~Note that, for some reason, the deleting visual effect doesn't work if there's only one analysis in the browser cache. Pressing the delete button does trigger the removal of the IndexedDB entry but it doesn't remove the entry from the UI until I refresh the page.~ Fixed.

[LTLA]

~Note to self: need to fix the handling of the file references when deleting analyses.~ Fixed.

[jkanche]

looks like most things work, haven't noticed any issues.