fixed issue https://github.com/globaleaks/GlobaLeaks/issues/86

@vecna @fpietrosanti @hellais 

this pull request has a twin sister on GLClient and both must be merged alltogether.

this series of commit fix https://github.com/globaleaks/GlobaLeaks/issues/86

now we need two variable to set the size limit: one for the json requests (up to 4KB?) and one for the file upload (10GB?)

@vecna can you take care of this? plese look at lines 116-123 of:
https://github.com/globaleaks/GLBackend/blob/87a10c060e5b0f36eeb064f9094afd7f06759c8b/globaleaks/handlers/base.py#L116