feat: Add gold members on pages foundation and community

[antoinehashbang]

About this issue: #2302

• Add a templatetag "gold_corporate_members", similar to the one used in the /download/ page
• In files "base_community.html" and "base_foundation.html", add a block "content-related-extra" in block "content-related"
• In files "foundation.html" and "index.html" (template for community page), use this block to call templatetag "gold_corporate_members"
• Add few tests for this new templatetags
• update test "test_community_index_number_of_queries" because we have increase the number of queries on this page

[sarahboyce]

Thank you for the PR! This looks great 👍
There's an update on the issue that we should have Gold+ sponsors (#2302 (comment)), do you mind updating?