Improve meta image size handling

[adrien-delhorme]

Description

Use a thumbnailer on the meta image field, that uses a BLOG_META_IMAGE_SIZE thumbnail option setting with a default size of 1600x1600px (facebook recommended size)

References

Fix #718

Checklist

• [x] I have read the contribution guide
• [x] Code lint checked via inv lint
• [x] changes file included (see docs)
• [x] Usage documentation added in case of new features
• [ ] Tests added

[yakky]

@adrien-delhorme I agree we need this, I'd ask to changes the behavior to set the default value for BLOG_META_IMAGE_SIZE to None and the method should fallback to the current behavior if the setting value isNone

[yakky]

@adrien-delhorme develop is now back in shape and pipeline is working good, could you please update this PR? thanks a lot

[yakky]

@adrien-delhorme we cherry-picked the relevant commit with #743 can you make a quick review of the new PR?

[adrien-delhorme]

I found a tiny typo in a code comment, but it is not important. Ok for this one! Thanks again! 🙏