Enable export to PDF and copying of attachments

[tagatac]

Use wkhtmltopdf for PDF support because it has the easiest emoji solution (via twemoji).

heic2jpg.go is untested as it is largely copied from upstream, and I have a PR open to create an interface upstream: adrium/goheif#2

goheif also has issues vendoring because there is no Go code in some of the required C++ subdirectories: golang/go#26366. This should also be fixed upstream, but as a stopgap, we need to use the cp and chmod commands (vendor makefile target).

Closes #4
Closes #5
Closes #7

[codecov]

Codecov Report

Merging #11 (60672cc) into main (336899a) will increase coverage by 1.62%.
The diff coverage is 91.85%.

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   90.44%   92.07%   +1.62%     
==========================================
  Files           3        4       +1     
  Lines         314      530     +216     
==========================================
+ Hits          284      488     +204     
- Misses         28       37       +9     
- Partials        2        5       +3     

Impacted Files	Coverage Δ
opsys/outfile.go	88.00% <88.00%> (ø)
chatdb/chatdb.go	98.29% <92.68%> (-1.71%)	⬇️
main.go	85.00% <93.39%> (+9.59%)	⬆️
opsys/opsys.go	96.96% <93.75%> (-3.04%)	⬇️

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