Allow manual symbol overrides from the SecurityService

[dickwolff]

For example IE00B3RBWM25 is VWRL. As a Dutch customer, I purchase it at Euronext (AS). But Yahoo finance result is VWRL.L (in GBp). Same thing goes for IE00B02KXK85, which should be FXAC.AS.

Proposed solution is to provide a file with key value pairs in which users can provide their own overrides. This file (if present) is then read in security service (similar to the cache setup) and used instead of any Yahoo Finance result.

overrides.txt

IE00B3RBWM25=VWRL.AS
IE00B02KXK85=FXAC.AS

File must also be volume mounted via Docker, if the user wants to use it.

[dickwolff]

This PR is waiting for #148 to be merged first.