Add screenshot generation to GitHub Actions pipeline

[teolemon]

• Add screenshot generation to GitHub Actions pipeline
• Upload generated screenshots somewhere (not directly the Play Store, since post-processing and validation is needed, a bucket or the OFF server will do)

          steps:
            - name: Upload generated screenshots as build artefacts
              uses: actions/upload-artifact@v2
              with:
                name: homework
                path: math-homework.txt

[Poussinette]

Hi! I'd like to work on this issue :)

[Poussinette]

@teolemon do you know which workflow should generate screenshot? and after which step should we generate screenshot and upload it?

[teolemon]

hi/salut @Poussinette
here are the screenshots we currently generate: https://photos.app.goo.gl/3KvmivgiXTsbzyMVA

• History (prepopulated)
• Scan (populated)
• Onboarding slides
• Product page (summary) with a bug on the front photo

Screenshots that would be nice:

• Adding compare mode screenshot with 2 products side by side
• Fixing the photo issue in the product page screenshot
• Search results screenshot

[teolemon]

@Poussinette Also, I think we should have a separation action, that would be triggered only on a specific branch ("generate-screenshots"). Then whenever we want to trigger a new generation, we would merger develop into this branch

[teolemon]

@Poussinette fyi, I created a draft PR for this, which is not working yet, but seems a good start