Use hash function for generating names of referenced resources instead of UUID.

[Svilen Ivanov]

What happened:
UUID is used when documentation nodes are referencing a resource (like images). This leads to a couple of problems where:

• if the resource doesn't exist the link is still overridden but loses its meaningful name
• if a second Docforge build is run the images have a new name

A solution to this problem might be hashing the resources' names in combination of their source location.

What you expected to happen:
to have reproducible and meaningful names for downloaded resources.

[Kristian Zhelyazkov]

/assign