Add error classes, docs and option to change add button text

...y/funds/migrations/0076_change_multi_input_button_text.py

...rojects/migrations/0035_change_multi_input_button_text.py

hypha/apply/stream_forms/blocks.py

hypha/apply/stream_forms/models.py

hypha/apply/templates/forms/includes/multi_input_field.html

hypha/static_src/src/javascript/apply/multi-input-fields.js