Show Originals: Improve option to hide only reblogs you follow

[marcustyphoon]

Description

One of those PRs that takes much longer to understand than to write :D There may be a way to refactor this or comment this to make it clearer.

The "always show reblogs of blogs I don't follow" option of Show Originals—which is probably better stated, at least for the sake of explaining this PR, as "only hide reblogs of blogs I follow"—is intended to ;00

...I spilled something on my laptop. will finish this later.

—is intended, as per the original requesting issue #1386, to make the feature hide "duplicated" content, i.e. reblogs which will appear in essentially the same form on the user's dashboard chronologically before the post in question. A reblog of a blog the user follows fulfills this condition.

This condition is also fulfilled if the direct reblogged-from post isn't from a followed user, but the last trail item is. This PR expands the definition of "reblogged from following" in this feature to include the last trail item being "from following."

Resolves #1726.

Testing steps

• Find a post reblogged from an unfollowed user whose last trail item was written by a followed user. Confirm that a) it logically would have been on the dashboard previously, and b) it is hidden by enabling Show Originals with the "Always show reblogs of blogs I don't follow" option enabled.
• Confirm that the option generally works as expected.