feature: better theming backend support

Now we can override ANY template, not just the ones in themed/ and/or static_views/ dir.

example:  
```
templates_themed/preregistration/disclaimers.html
``` 
if it exists, overrides:
```
templates/preregistration/disclaimers.html
```

real work:
- created a new dir named templates_themed/, put some magstock stuff in there.
- setup configspec.ini to setup TEMPLATES_DIR