gbrowse2 packaging

[Olivier Sallou <olivier.sallou@irisa.fr>]

Hi,
I have updated gbrowse packaging.

I fixed licensing issuess: dropdown css are GNU GPL (copyright updated) 
and I remove contrib directory from source via a get-orig-source target.
"Contribs" are not required for software.

Could you have a look at packaging for upload ?

Thanks

Olivier

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