Upgrade to Wagtail 6.3 and wagtail-localize-smartling 0.5.0

[stevejalim]

...plus some general subdeps.

Resolves #15368

Notes

• Wagtail 6.2 release notes: https://docs.wagtail.org/en/v6.3/releases/6.2.html
• Wagtail 6.3 LTS release notes: https://docs.wagtail.org/en/v6.3/releases/6.3.html

Note that Wagtail 6.3 adds a 'description' field to image models, which we won't automatically surface in templates, but could/should as alt text, if we agree with how it's been done.

Testing

• Pull the branch
• Copy your data/bedrock.db somewhere safe if you need that state
• make preflight -- --retain-db to install deps and run 6 migrations from Wagtail and two migrations from Bedrock
• Test drive locally, adding CMS pages and images, etc

[robhudson]

r+wc

[stevejalim]

Surpassed