Replace Donate page with more extensive Funding page

[Calinou]

This is a preliminary version of the Godot Development Fund page I've been working on as part of my contract.

Other changes done as part of this PR:

• Replace the Patreon bar with a simpler highlighted Donate button.
• Move the list of sponsors to a reusable template to avoid duplication across pages.
• Add more utility classes and move some of them to main.css.
• Update the Patreon plugin to return the number of patrons and the unformatted amount of money in cents. The number_format Twig filter is more appropriate for number formatting.

The number of corporate sponsors and amount of money donated outside Patreon should be configured here: https://github.com/Calinou/godot-website/blob/75a5380cdf81ecdfae2be199c9fa7fe78bbc7f54/themes/godotengine/pages/fund.htm#L8-L21

This closes https://github.com/godotengine/godot-website/issues/102.

Preview

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Grants

WIP: Needs more information on ongoing and past contracts to make this page more complete.

Corporate memberships

WIP: Needs more text.

[Calinou]

Some updates:

• Rebased against the latest master branch.
• Added automatic dark theme support and made sure that new all pages look correct with both themes.
• Updated the Grants page with the latest hires.

[coppolaemilio]

Considering the branch conflicts and that we are going to be refreshing this among other pages, should we close this PR @Calinou? I think the work done can be a great start for the updated page but not sure if we should keep the PR open. What do you think?

[Calinou]

Closing per @coppolaemilio's comment.