Display image sizes in extensions manager

[Williangalvani]

No description provided.

[ES-Alexander]

This is kind of handled by the disk usage indicator for running extensions, but it would be preferable if we could also display things like the space taken up by old (still installed) versions, and by data / logs stored in the filesystem (per #1640).

Something like one of these could be useful, although given we want a webUI we might want something like diskover. Alternatively we could roll our own with the specific categories and groupings we're interested in - not sure how involved that would be.

[ES-Alexander]

Adding to this that it would also be useful to show the size of extensions that have not yet been installed.