Local charms icon and readme

[Francesco Banconi]

The icon and meta information about a local charm not yet included in the charm store are not displayed correctly.

To dupe:
- juju quickstart an environment;
- deploy a local charm not having a counter part in the charm store. This can be done, for instance, by downloading a charm archive from the charm store (https://api.jujucharms.com/charmstore/v4/trusty/mysql/archive), unpacking it in a local repo and contextually renaming it: e.g. to ~/cs/trusty/foo. Then change the name of the charm (included in its metadata.yaml) to "foo" and deploy the charm with "juju deploy --repository ~/cs local:trusty/foo;
- visit the GUI: the charm icon should be properly displayed in the service block in the canvas;
- double click the service block, and note that the default icon is instead displayed in the entity inspector;
- also note that the charm panel does not work, and the spinner never goes away.

In essence, we need to fix the charm panel and the inspector local charm icon.

The GUI sends a request to https://manage.jujucharms.com/api/3/charm/trusty/foo-22 which is obviously wrong.
The charm info should be retrieved from the Juju API itself for local charms, as we cannot assume they exist in the charm store. We should also make sure that the GUI no longer connects to the old API v3.