specify the format for dateUploaded in categorize_dataset

[laijasmine]

While working on the reporting function @jeanetteclark found that the dateUploaded column is inconsistent in the datset categorization sheet where sometimes rows are added in this format: ’2021-01-21 17:17:36 and other times it is added in this format: 2/9/2021 0:52:06 . Because of this we need to first read in the data as character before converting to POSIXct.

It would be helpful to specify the format when using googlesheets4::sheet_append(). However this isn't available at the moment: tidyverse/googlesheets4#81