Fix image attachment bugs

[ath92]

Found two bugs with image attachments:

• single-attachment images didn't show up in the lightbox
  • (found out that we were setting both source and src for multiple-image galleries, but only source for single-attachment galleries, the latter seems to correspond to react-images types)
• after clicking a thumbnail in a gallery with multiple images, it was impossible to close the lightbox

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Description of the pull request

[codecov-io]

Codecov Report

Merging #795 (d9d10bd) into typescript-conversion (1e2b96f) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@                    Coverage Diff                    @@
##           typescript-conversion     #795      +/-   ##
=========================================================
- Coverage                  84.51%   84.51%   -0.01%     
=========================================================
  Files                        189      189              
  Lines                       4669     4668       -1     
  Branches                    1441     1440       -1     
=========================================================
- Hits                        3946     3945       -1     
  Misses                       523      523              
  Partials                     200      200              

Impacted Files	Coverage Δ
src/components/Gallery/Gallery.tsx	88.00% <0.00%> (-0.47%)	⬇️
src/components/Gallery/ModalImage.tsx	100.00% <ø> (ø)

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