Correctly detect the number of article for zim version <= 6

[mgautierfr]

Fix kiwix/kiwix-tools#536

[codecov]

Codecov Report

Merging #743 (85a9d35) into master (a17258f) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #743      +/-   ##
==========================================
+ Coverage   60.47%   60.55%   +0.08%     
==========================================
  Files          56       56              
  Lines        3724     3732       +8     
  Branches     2058     2061       +3     
==========================================
+ Hits         2252     2260       +8     
  Misses       1471     1471              
  Partials        1        1              

Impacted Files	Coverage Δ
src/book.cpp	86.66% <100.00%> (ø)
src/tools/archiveTools.cpp	87.01% <100.00%> (+1.50%)	⬆️

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[kelson42]

@mgautierfr I don't think this is proper. We need to reintroduce the media and article count based on M/Counter... this is what was done earlier.

[mgautierfr]

This is what is done with (old) zim files (<=6). For new zim files, we use the list of front articles (the same on which we do random or title suggestion).
Using the front articles list is probably more accurate as we don't count text/html items with are html redirects (warc2zim).