Use icons from supported apps when configuring a tag #532

When adding an entry/item that is a supported app (e.g. Syncthing) Heimdall is very helpful in auto-suggesting an icon.
The same behaviour would be great when creating a new tag. For example, I have a tag combining all my Syncthing URLs, but if I want to have the Syncthing icon applied to the tag I need to manually download the icon from one of the individual entries and then upload it for the tag.

It would be nice if Heimdall behaved the same way for tags as it does for individual items, suggesting icons from the supported apps in addition to being able to upload a custom icon.