[Gutenberg] Build a base layer for Homepage

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As an user of the Travel theme, I should have a way to manage blocks.

This case creating a skeleton/base for all of these blocks that as a placeholder would just display the content from the current homepage template could be perhaps done in one ticket, and then these blocks could be made configurable and functional one by one in separate tickets.

This skeleton layer should allow use of these blocks:

• [Gutenberg] Build a hero block for homepage #14 Hero block (consists of h1 and h2 subheading, Search input; datepicker; button)
• [Gutenberg] Build a "Discover" block for Homepage #20 Discover / display post block
• [Gutenberg] Build a "Secondary Header" block for Homepage #21 Secondary header (consists of h2 and div subheading)
• [Gutenberg] Build a "From the blog" block for Homepage #22 From the blog
• [Gutenberg] Build a "Browse by activity" block for Homepage #24 Activities list (Browse by activity)
• [Gutenberg] Build a "Top adventures" block for Homepage #23 Top adventures
• [Gutenberg] Build a "Featured Destinations" block for Homepage #25 Featured destinations
• [Gutenberg] Build a second search block for Homepage #26 Second search block (search input + button/link)

And the homepage should be styled (#35).

As requested (and detailed) by @miina here.

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Also, I believe we'll probably do one of these for the CPT, too!