Dashboard: add preemptableJobByDefault cluster config.

[Gerhut]

• Should be Boolean
• Backend: fill unset preemptionAllowed with set preemptableJobByDefault
• Frontend: in submittion page, automatically fill preemptible field with set preemptableJobByDefault when cluster changed

[coveralls]

Pull Request Test Coverage Report for Build 3819

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 92.249%

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Totals
Change from base Build 3815:	0.04%
Covered Lines:	988
Relevant Lines:	1060

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💛 - Coveralls