Three minor fixes in scss

When I tried the command 

```
sass --watch plonetheme/onegov/resources/static/static.scss:plonetheme/onegov/resources/static/static.css
```

I saw two errors and a deprecation warning, which I fixed here in ```directory.scss```, ```form.scss```, and ```static.scss```.  The file ```static.css``` was just a result of the recompilation, but apparently it has not been updated in a while, so it's pulling in a whole bunch of other older changes.

I'm a complete sass/scss n00b, so please let me know if these changes would cause any problems.
I have 

```
> sass -v
Sass 3.3.10 (Maptastic Maple)
```
on a Mac.