Epic: refine validation

[praseodym]

Refine data entry validation to implement all documented rules, show copy to the user, and make sure all of this is tested.

• [x] #135
• [x] #188
• [x] #136
• [x] #149
• [x] #85
• [x] #86
• [x] #185
• [x] #150
• [x] #151
• [x] #186
• [ ] #271

[jorisleker]

One thing to note regarding copy of the validation messages. The messages as defined in the docs are aimed at Invoerders. Some validation messages will be shown to the Coordinator as well. We'll need slightly different copy for that role. Something to take into account when setting up the translation file I guess.