Expand coverage of stable mailing list

Right now the project only looks at emails from the stable mailing list with subjects starting with "[PATCH]". This means various patches are ignored:

• Patch submissions with CC: stable
  • Patches from patch series with the format "[PATCH x/N]"
  • Patches that have gone through several revisions with "[PATCH vN]"
  • Patches with branch specific tags like "[PATCH -net]"
  • "[RESEND PATCH]" / "[PATCH RESEND]"
• Backport requests
  • "[v4.19-stable PATCH]"
  • "Please consider mainline commit <hash> for stable"
• Stable RC commit reviews, aka "[PATCH 5.3 xx/NN]"
• Patches from Sasha's automatic selection, aka "[PATCH AUTOSEL x.y xx/NN]"

We should expand the coverage to look at some or all of these.