Add support for bookmark thumbnails (#721)

* Preview Image

* fix tests

* add test

* download preview image

* relative path

* gst

* details view

* fix tests

* Improve preview image styles

* Remove preview image URL from model

* Revert form changes

* update tests

* make it work in uwsgi

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Co-authored-by: Sascha Ißbrücker <sascha.issbruecker@gmail.com>

.gitignore

@@ -194,3 +194,5 @@ typings/
 # ublock + chromium
 /uBlock0.chromium
 /chromium-profile
+# direnv
+/.direnv