Download zip in charm detail pages could require discharge

[frankban]

When charms/bundles are gated or require terms, just linking a charm store resource fails, because a full bakery authentication is required. For instance authenticate, go to https://jujucharms.com/u/cmars/mattermost/16 and click "Download .zip" on the right panel.

[clagom]

@hatched @huwshimi Is this still relevant?

[hatched]

It is.