Add extra data validation checks to IATI Publisher data entry forms

[emmajclegg]

Following completion of #1553 (improved data entry forms) and discussion on #1559 (element-level data completeness labels), this issue is about adding additional data completeness checks to IATI Publisher's data entry forms.

User story: As someone entering data, I want to be prevented from saving a data entry form if I have missed mandatory information in it, so that I can fix this straight away.

Considerations:

• Some data checks already exist in IATI Publisher's data entry forms - e.g. logic ensuring that recipient-country and recipient-region percentage rules are respected.
• IATI Publisher currently lets a user save a form if a sub-element has been only partially completed (e.g. location location-id @/code has been populated but not @/vocabularly) - this is an example of something best checked at the data entry form level
• Element-level "completed" vs. "not completed" labels in IATI Publisher currently check whether all mandatory sub-elements have been populated.
• We don't want data entry forms being overly strict - i.e. a user may want to part populate an element, save and come back to finish it.

Suggested order of priority for elements:

• participating-organisation (one of narrative or reference needed) - completed in #1596
• receiver-org and provider-org for a transaction - same changes as #1596 to be applied
• location sub-element location-id - @/code and @/vocabularly need populating if sub-element is completed