Add icon for external storage folders to breadcrumbs

[MorrisJobke]

See discussion here: #5918 (comment)

[jancborchardt]

Yep. Tagging as good Junior Job opportunity. :)

[PVince81]

On master the new breadcrumb implementation allows for setting icons on any section, so implementing this should be even easier now 😄

But one challenge is to for the files app to get the file_info for every section of the path.

[jancborchardt]

So anyone up for adding it? @owncloud/designers

[jbtbnl]

Why do we need the icon? There should be no difference in user experience between local and external folders.

[jancborchardt]

It’s informational, to know it’s an external folder – and which folder in the breadcrumb the root of that external mount is.

Similar to the »Share icon in breadcrumbs«: #4333 (which should include the possibility to share the current folder)