[FIXED JENKINS-12379] Allow specifying a context for archiving

**Please provide feedback**: What's a user-friendly name for this new option? *Search Context* seems weird but I cannot think of anything better.

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Possible unexpected side effect is that the fingerprinted path to files is also relative to the search context. I think it makes sense to have it mirror the structure of the archived artifacts.

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Screenshots of the new option and form validation below. Note that some minor wording changes have occurred since these screenshots were taken.

**Trying to escape the workspace**:
> ![screen shot 2014-12-20 at 17 32 25](https://cloud.githubusercontent.com/assets/1831569/5515789/4d0701fc-887b-11e4-9e7f-9b9a00061345.png)

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**Workspace exists, but specified context does not**:
> ![screen shot 2014-12-20 at 17 31 42](https://cloud.githubusercontent.com/assets/1831569/5515792/4d0bb558-887b-11e4-9fbb-1ca432636a4e.png)

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**Existing validation for artifact pattern (and suggestion) recognizes current value of context**:
> ![screen shot 2014-12-20 at 17 31 24](https://cloud.githubusercontent.com/assets/1831569/5515791/4d0ac45e-887b-11e4-9c19-4e2549c73c2c.png)

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**Input can be validated based on workspace contents**:
> ![screen shot 2014-12-20 at 17 31 34](https://cloud.githubusercontent.com/assets/1831569/5515795/7089d9b0-887b-11e4-8be9-5630e5948e20.png)