Sandbox mode local charms upload preparation.

Sandbox mode local charms upload preparation.

Code reorganization to introduce a fake web handler
which will simulate the local charm upload process in
sandbox mode.

QA:
1) Bootstrap juju core trunk and deploy the charm with
this branch. One way to do it is:
juju bootstrap --upload-tools
juju deploy juju-gui
juju expose juju-gui
juju set juju-gui juju-gui-source="https://github.com/frankban/juju-gui.git local-charm-prepare-fake"
Another way to do the same is:
create a release (with BRANCH_IS_GOOD=1),
manually include it in a local the gui charm,
remove the stable release already in the charm,
and then run "make deploy".

2) Open the GUI, login and deploy a local charm
(zip drag and drop to the topology).

3) Ensure everything works well and deploy the local charm.

4) Now let's do the same in sandbox mode
(juju set juju-gui sandbox=true) or just "make prod"
in the branch.

5) This time, after dropping the zip, you should see
a notification message like
"local charm upload in sandbox mode not yet implemented".

6) Done. Destroy the environment.