CSV upload improvements

[rw251]

The recent RSI iteration was on how people upload CSVs and whether the default of excluding all missing descendants was contributing to low quality codelists. This PR is the final interface following a few rounds of user testing.

1. Selecting a CSV on the create codelist page, triggers an async request to a new csv-descendants-preview endpoint
2. This analyses the CSV and:
   • detects the coding system
   • detects if there are header rows
   • detects the column containing the clinical codes
   • finds any codes, not in the CSV, but where they have an ancestor code in the CSV
3. The user is then presented with the results
4. If there are "missing" children, the user is shown which codes are missing, and a choice to exclude them all (the current default), or to leave them unresolved and decide on the next screen (the builder page). The default is to choose case by case as this is the safer option

The benefits are:

• More user friendly:
  • the user no longer needs to specify if there is a header row because it's detected automatically
  • the user no longer needs to have the clinical codes in the first column because we detect the column
  • various checks are done as soon as the user selects the CSV and warnings/errors can be displayed before they submit the page by clicking Create
• Makes it harder for a user to accidentally create a codelist with missing children, while still retaining the option to create a CSV codelist in the previous way (by excluding all missing children)

Selected screenshots

1. Removed the "does your file have a header" radio. Updated help message to explain what will happen:

2. Display preview of CSV along with what was detected:

3. Check if detected coding system clashes with selected coding system:

4. Existing errors with CSVs can now be shown immediately. Previously this warning would only appear after page submission:

5. If there are missing children, we explain the options, and show which codes are missing:

6. Excluding missing child codes has a visual indicator in the list of missing codes:

[rw251]

The following issues can likely be closed when this change is merged:

• Provide some way for users to select the column containing the clinical codes. #2694
  Autodetection of coding column means no need to add a way for users to pick their column
• Improve the handling of uploaded csv files #2655
  The parent ticket of the above
• Make errors more visible for CSV upload on the "create a codelist" form #2800
  Errors now display as soon as you upload the CSV
• Make behaviour less surprising for CSVs that fail to upload via "Create a codelist" form #2801
  Not allowing the create button to be clicked until the csv is valid prevents this edge case