Fix for SWC-147 - Alert display offset
Fix for SWC-147 - Alert display offset

Changed css to display "global" alerts at a fixed screen position
so the alert is visible regardless of where the user is scrolled to.

Sage-Bionetworks/SynapseWebClient,jay-hodgson/SynapseWebClient,jay-hodgson/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,jay-hodgson/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,Sage-Bionetworks/SynapseWebClient,jay-hodgson/SynapseWebClient