Move client code into a separate directory

See #209 for context. This pull request moves all client code (templates, JS, images, etc.) to `/adhocracy_client` in order to facilitate the development of other clients, allow easier experimentation with the whole client, and prepare for the #182, the move to Pyramids.

Currently, legacy methods are inserted in order to make sure that nothing breaks. We can modify the legacy controllers one-by-one without breaking anything else.

As an example, the admin controller has been changed to the new format. In most cases, the changes are minimal. Basically, we replace

```
from pylons import tmpl_context as c
c.x = 'foo'
c.y = 'bar'
render('baz/beat.html')
```

with
```
data = {
    'x': 'foo',
    'y': 'baz'
}
render('baz/beat.html', data)
```

This fails when code assumes that it can set something in `c.` and keep that for quite some time. Currently, only the `TimerProxy` in adhocracy/config/environment.py (wtf is that doing in *there* anyways?) seems to be problematic.