fix unwanted copies of return values

[maxbachmann]

These functions are supposed to return a const reference instead of a copy that isn't allowed to be modified.

[maxbachmann]

Edit: Thinking about this a bit more this was probably supposed to guard against the config hashmap getting modified.

I cleaned this up in a couple of ways:

• return by value instead of const value
• remove outdated static empty string that doesn't serve any purpose anymore
• avoid unnecessary copy of the config hashmap when getting the log / shared content directory

[maxbachmann]

ping @tculotta @dgkanatsios @nassosterz-ms