Combine read and write access in the workflow editor

[matthias-ronge]

In the workflow editor, read and write access can be configured independently for the user in one workflow step. This is incorrect because

• either no access is granted,
• or read-only access
• or read-write access.

The user interface must be adjusted here to reflect this (e.g. 3-fold drop-down).

[matthias-ronge]

This also affects the data model, where these are two different boolean values, but should be enum:

    @Column(name = "typeImagesRead")
    private boolean typeImagesRead = false;

    @Column(name = "typeImagesWrite")
    private boolean typeImagesWrite = false;

[matthias-ronge]

The two tasks are separately described in #4448 and #4449.