User.config file path should have the name of the add-in's assembly

As part of investigating #143 I've noticed that the ExcelDna's assembly is being used as evidence for the `user.config` file, and therefore any ExcelDna add-in `user.config` is written to a path that looks like `%localappdata%\Microsoft_Corporation\FullTrustSandbox(Excel-DN_Path_jojpsvsna1vaszvd35jsryd0vnvv001i\16.0.8431.2079`.

This means that **different add-ins, running in different AppDomains, etc. will all share the same `user.config` file, instead of having their own**.

We should consider implementing our own `HostSecurityManager` or our own `SettingsProvider` and customize this path, using the assembly evidence from the main add-in assembly, instead of using the ExcelDna's one.