Minor cleanup
Add Item.IsSummary property for ComicBookSeries

This read-only property will allow ComicBookSeries instances to be
created in 'summary mode', meaning that they do not have to have
to contain entries.
Refactored ComicBookSummary page logic

Previously, EntiresCount was populated by a 'SELECT COUNT(*)' from
ComicBookIssues query that run for each comic book. This meant two
things:
1. Pagination method runs 1 + N queries to get everything
2. ComicBookSummary could not be created with its count.

1 meant that the whole thing was slower than it could be.

2 will be important at a later time because I want to replace
ComicBookSummary with a summary-mode ComicBookSeries to better leverage
generic classes & methods.

I also replaced a Inner Join/Where combo with Inner Join ON. This might
have slight, negative performance consequences but looks nicer.