Feature request: Classification of places as "optional"

[JudithBirke]

Current state:
Activities can be starting points and/or mandatory to get to another node. They can be targets and compulsory to complete the learning map activity itself. The different states are indicated in different ways. A distinction between recommended, supplementary, and compulsory activities is missing.

Requested state:
Activities can be classified as optional. For example, a dotted line could indicate this state.
This way, activities with further practice exercises, supplementary material, etc. can be identified as such prior to opening the activity and can be distinguished more easily from unvisited mandatory places.