Unable to override Pager.RecordsPerPageUrl()

Hi,

This is a really small issue. The ["Pager" class](https://github.com/HangfireIO/Hangfire/blob/master/src/Hangfire.Core/Dashboard/Pager.cs) has two methods to build the URL for each pager link:

```cs
public virtual string PageUrl(int page)
{
if (page < 1 || page > TotalPageCount) return "#";

return BasePageUrl + "?from=" + (page - 1) * RecordsPerPage + "&count=" + RecordsPerPage;
}

public string RecordsPerPageUrl(int perPage)
{
if (perPage <= 0) return "#";
return BasePageUrl + "?from=0&count=" + perPage;
}
```

IMHO, the method "RecordsPerPageUrl()" should be "virtual" too.

I was trying to inherit this pager to create a new one where I can include custom querystring parameters. This approach works OK for "PageUrl()" override, but it's actually impossible to complete the inherited class, because I'm unable to override "RecordsPerPageUrl()".

This means I can only customize the bottom pager, but not the top one where you select the number of records per page.

Start with src/Hangfire.Core/Dashboard/Pager.cs and compare RecordsPerPageUrl(int perPage) with the existing virtual PageUrl(int page) method. Make the records-per-page URL customizable for inherited pagers, and verify that the top pager can preserve custom query-string parameters as described in the issue.

Written by the indexing model from the issue text.