Add Field Interface

[M-C-McGrath]

Users should be able to add their own fields to documents. This is already supported on the backend, but there needs to be an interface which collects the following details:

• Document type to add field on
• Field name (human readable name)
• Icon
• Field type

[bnvk]

@Shidash one question, does the functionality here add the field to the dataspec (thus see across all documents in Edit Document functionality)? Where do you suppose is best place to do this? I see two intuitive options:

1. In the Show document view connected to Edit Document functionality
2. At a higher level of defined dataspecs or such where it is a sort of meta-editor

I fear that 1. runs the risk of adding too much complexity to editor codebase + UI / UX of what is communicated to user.