fix(blog): correct inverted last_page context value in blog_index

[SAY-5]

Problem

blog_index in src/blog/views.py populates last_page in the
template context with page.has_previous():

context = {
    ...
    "last_page": page.has_previous(),
    ...
}

Paginator.Page.has_previous() returns True whenever there are
pages before the current one — i.e., for every page except
page 1. The variable name promises the opposite: last_page should
be True only when the reader has reached the final page.

Downstream templates use last_page to terminate the "load more"
cycle / show an end-of-feed marker. With the old value the feed
appeared to end as soon as the reader clicked past page 1, regardless
of how many posts actually remained. #857.

Fix

Use the semantic opposite that actually matches the variable name:

- "last_page": page.has_previous(),
+ "last_page": not page.has_next(),

page.has_next() returns False only on the final page, so
not page.has_next() is True exactly when the reader has reached
the end of the feed.

Fixes #857