Combine database colums for read images and write images

[matthias-ronge]

See also #4448.

There are two boolean database columns of tasks typeReadImages and typeWriteImages, and corresponding properties of the task object. However, this reflects three states:

• No link is created in the user directory when the task is taken over.
• A link is created in the user directory when the task is taken over, but the directory and the images belong to root and can therefore not be changed (e.g. for quality control)
• A link is created in the user directory when the task is taken over, and the directory and any images belong to the user and can be added, changed or deleted (e.g. for scanning)

These should be reflected by an enum with three options in Java code and database.