RoE functionality in Platform

[benhayes21]

Issue Description

the oasislmf platform now allows for currency conversion but we need to add the functionality into the platform to allow users to access upload RoE tables at platform level and utilise them in anlyses

[sambles]

• Platform wide (its own endpoint holding multiple exchange rates)
• Option to select and attached exchange rates to an analysis
• Add option to set one of the above as default which is automatically attached to each analysis

[sambles]

Question: where to put input for 'reporting currency' --> add to portfolio