Customizing the error grid page

[IhorSysak]

Hi! I have made a PR to customize your 'ErrorList' page since in my prod project we use a tenant approach. It means that we have several tenants and appropriately we want to separate their logs, to hide logs which don't belong to this tenant. Also, I added the ability to remove columns which are useless for us from the error grid and add additional columns which are needed. For this, I was forced to change the src/StackExchange.Exceptional.Shared/Pages/ErrorListPage.cs to make it more flexible. Hope you will like it. I'd be happy to talk to you if you have any questions about this!