More CI features

Feature requests:

• Fail on absolute paths to the GFX in SVGs. They should be relative (proposed by @Deevad in !133 (comment 320666))
• Fail on (or even autocorrect) invalid paths: ..\gfx….png → ../gfx….png (proposed by @Midgard)
• Fail on invalid XML syntax for any SVG file (not just cover/title, proposed by @calimeroteknik)
• Fail if .gitkeep files are present (proposed by @Midgard)
• Give feedback on MRs (probably a bot that leaves a comment on it)
• Don't install Python from scratch each time (!145 (merged))
• Fail (or just notify maintainers) if the info.yaml does not conform to the info.yaml spec (proposed by @Midgard)
• Fail (or just notify maintainers) if any ep ≥ 26 has a translation without metadata in info.yaml (proposed by @Midgard)
• Also run on new-epXX_… episodes (proposed by @Midgard on IRC based on bf069308, added here by @xHire)
• Check spacing, notably before punctuation that has rules for that (in French, a thin space: grep -E '[^"<>?! ][?!]|«[^ ]|[^ ]»' *.svg) (proposed by @Midgard and @calimeroteknik)

[Added by @xHire]
Investigate whether it’s possible to:

• Turn off environment setup output (at least when everything is OK) → Not needed any more. The CI now leaves a comment.
• Turn off CI e-mails within the repo/project → Not possible. There's an option in the user settings but that requires the translator themself to change it.