FTT Tax validations for mandatory fields 

Partially completed, with the following missing things and remaining fixes:

1. Judges validation is not implemented yet.

2. Subcategories validation is not implemented yet.

3. When mandatory fields are missing, the names of the fields in the error messages are not correct:
"Claimant" should be "Appellant".
Maybe "Doc file" should have better label?
"Category decisions" should be "Categories"

4. If you submit the form with a selected doc file but with some mandatory field value missing, the selected doc file value is lost. So the doc needs to be attached again.

5. If all mandatory fields are filled, sometimes the upload takes a long time and then gives an error. This is probably because the conversion is taking too long. If you go back to the homepage and reload it, the new decision does appear. So it does get saved, but you get an error because of waiting too long for the conversion. We should ideally move the doc processing into a background process.

@danhawkins @jairodiaz Merge or not based on discussing the above with Dani. Also, feel free to make any changes and fixes to this branch.