Fixes 'Find it' link errors for non-Green locations

Closes #733 

Moved `has_stackmap?` validation from Callnumber class to helper.

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![image](https://cloud.githubusercontent.com/assets/302258/3940082/b40c7cc0-24d4-11e4-8f38-d8e6458e13d9.png)
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![image](https://cloud.githubusercontent.com/assets/302258/3940081/a324faae-24d4-11e4-9db1-479d3965e42e.png)
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![image](https://cloud.githubusercontent.com/assets/302258/3940098/19d14ed2-24d5-11e4-8ce2-757e8d63839a.png)


@jkeck Moved 'Find it' link to the end based on Jennifer's comment (#733 last screenshot). Will move it back to front after her confirmation. Also, 'Find it' link needs styling to differentiate it from MHLD statements.