Use new FORUM_* config from API

[nicksellen]

What does this PR do?

Something broke in the frontend build so that it loaded the dev banner topic for dev and prod, I decided it makes more sense to use the server API config endpoint, so we can configure it using .env file on server, not have it baked in. Which further moves us to have a more generic frontend build, not one that requires build-time variables.

It uses the newly added FORUM_* config options that the API returns for:

• banner topic (default is 930)
• discussions feed (default is latest.json, but you can configure it for a single category, e.g. c/feedback-ideas/31.json -- so long as it has the same json structure as the other feeds (compared to latest.json))

[codecov[bot]]

Codecov Report

Merging #2662 (c7d2b51) into master (6094a56) will decrease coverage by 0.04%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #2662      +/-   ##
==========================================
- Coverage   75.88%   75.84%   -0.04%     
==========================================
  Files         384      384              
  Lines       41005    41009       +4     
  Branches     2361     2342      -19     
==========================================
- Hits        31116    31105      -11     
- Misses       9889     9904      +15     

Impacted Files	Coverage Δ
src/communityFeed/api/communityFeed.js	57.89% <50.00%> (-5.27%)	:arrow_down:
src/communityFeed/queries.js	100.00% <100.00%> (ø)
src/communityFeed/services.js	68.88% <100.00%> (-4.37%)	:arrow_down:

... and 1 file with indirect coverage changes

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