Specify relation column as required

Node.js version: 8.9.4

npm version: 5.6.0

Strapi version: 3.0.0-alpha.9

Operating system: Ubuntu 16.04 x64

Additional details -

Database being used: PostgreSQL 9.5.10

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
There is no way to specify a relation column as required.

If the current behavior is a bug, please provide the steps to reproduce the problem

1. Create a new model in Strapi.
2. Create a relation attribute in this model. Notice that there is no option for marking this column as required in advanced settings.

What is the expected behavior?
Relation columns should allow to be marked as required.

This may solve #556

[derrickmehaffy]

Can confirm, relations should have a required function.