Use CMS for Gallery and Springboard updates

[maureenlholland]

Description

These PRs are essentially content swapping work it would be nice to place in the hands of people who want to update the content:

• Home and about updates - WT-492 (fix #16915) #16927 (3 weeks after previous update)
• Homepage updates #16861 (1 month after previous update)
• Add mozfest block to homepage (fixes #16792) #16802 (time-sensitive update for MozFest)

Needs localization consideration for Wagtail CMS

If we ultimately decide not to use CMS for quicker updates, we should create an image sizer script to automate the work of generating all the responsive image sizes.

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Success Criteria

• Decision on whether to Wagtail these sections or not
• More specifically scoped tasks to simplify these content updates in future (whether Wagtail or image automation)

[maureenlholland]

closing in favour of #16980