Views: set mandatory-state of a column

[blizzz]

When using views for different stages of processes, some columns might deviate from the original table definition and turn mandatory.

• Needs: refactor: restructure column config to support additional settings #1688
• BE: handle storing, retrieving and updating the mandatory flag with the dedicated column content.
• BE: the create and update logic of rows in views need to take this setting into consideration
• FE: together with View feature: set permission per column: "writable" or "read-only" #1154 (comment) agree on an UI for such elements (this step does not need to wait for 1688.)
• FE: implementation of the results ^ and of course related business logic.