Issue/1938 reader update logic

Fix #1938 - resolved problems with missing posts, primarily seen in "Blogs I Follow," by no longer using the `&after` query param when requesting new posts (see #1938 for explanation). To avoid the problem with missing posts between usage gaps, older posts are purged so that only as many posts as requested in a single call are kept between usages.

This PR also changes how posts are purged so that (1) posts aren't automatically removed from "Blogs I Follow" every time followed blogs are updated, and (2) posts are now removed on a per-tag basis so that a max of 200 posts are kept for a tag.

Last but not least, this PR simplifies & cleans up the way posts were updated and the way new/changed posts were detected and reflected in the UI.