Add signed-url handling to thumbnail service

Co-authored-by: André Duffeck <a.duffeck@opencloud.eu>

Signed-off-by: Christian Richter <c.richter@opencloud.eu>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f