Add some more space to "Keywords" search field of User Search

In the User Search page, the label for the "Keyword" field is quite small. In some translation (for instance in Italian), the translation of Keyword is longer and the aspect is not so good.
So, lets
- widen the space available for the Keywords search field
- show "Advanced Search" instead of "Advanced" in link that shows Advanced search criteria (same text as File Manager)
- pull the whole search form down a bit to not make the Search button fall over the "Advanced Search" button
- make the space for Group field as wide as the Set field of file manager (so that future adjustments can be applied symmetrically to both layouts)

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Before:

![searchform-before](https://f.cloud.github.com/assets/928116/1334247/a13ae678-35a5-11e3-80f9-4dff9e5631ea.png)

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After:

![searchform-after](https://f.cloud.github.com/assets/928116/1334250/a8f8f0da-35a5-11e3-89e8-2abdd04f5657.png)