Improvements to image upload

[Uzay-G]

This implements a new config option, inside SCRAPING_CONF["save_images"] where images called in downloaded webpages are stored locally. I decided to set this to False by default.

[Uzay-G]

This is just a draft, I might try to add testing, and I also think that the code might be improved.

@srevinsaju I think you're right, this feature will be quite nice. Tell me what you think of the PR, when you can 😊

[codecov]

Codecov Report

Merging #225 (b03d95f) into master (019d033) will increase coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   82.40%   82.82%   +0.41%     
==========================================
  Files          27       27              
  Lines        1967     2026      +59     
==========================================
+ Hits         1621     1678      +57     
- Misses        346      348       +2     

Impacted Files	Coverage Δ
archivy/config.py	91.66% <100.00%> (+0.36%)	⬆️
archivy/data.py	90.86% <100.00%> (+0.69%)	⬆️
archivy/models.py	85.61% <100.00%> (+1.35%)	⬆️
tests/unit/test_models.py	100.00% <100.00%> (ø)
archivy/api.py	82.79% <0.00%> (-0.93%)	⬇️
archivy/forms.py	100.00% <0.00%> (ø)
tests/functional/test_routes.py	100.00% <0.00%> (ø)
tests/integration/test_api.py	99.21% <0.00%> (+0.03%)	⬆️
archivy/routes.py	88.88% <0.00%> (+0.15%)	⬆️

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