(incorrectly) limited link roles in personal space

[kulmann]

Steps to reproduce

1. Log in as einstein
2. Create a folder "from-einstein"
3. Share folder "from-einstein" with marie
4. Log in as marie
5. Visit "Shared with me" page
6. Accept "from-einstein" share
7. Create a quick link with viewer permissions (= default) on "from-einstein" (e.g. via right-click context menu)
8. Navigate to "Personal" space (still as marie)
9. Create a folder "my-stuff"
10. Create quick link on "my-stuff"
11. Try to edit the role of the quicklink on "my-stuff"

Expected behaviour

As "my-stuff" belongs to marie, all possible roles need to be available

Actual behaviour

Only the "Viewer" role is available

Bug analysis

FileLinks.vue has a computed prop share which only ever picks the first incoming share it finds in the root of the shares tree. This is wrong because the root contains more share entities than just the ones related to the given folder "my-stuff", in this case also the reshared folder "from-einstein". Because of that the link from (10) is identified as a reshare, which is wrong.