Catalog, Search: add help dropdown to the index landing page

[fiskus]

Search Help has a slightly different look for Header and OpenLanding. I made NavBar/Help.js more abstract (in terms of styles) and reused for OpenLanding. Maybe I should move it to another place, ex. components/SearchHelp.js

[codecov-io]

Codecov Report

Merging #1838 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1838   +/-   ##
=======================================
  Coverage   89.83%   89.83%           
=======================================
  Files          62       62           
  Lines        7387     7387           
=======================================
  Hits         6636     6636           
  Misses        751      751           

Flag	Coverage Δ
#api-python	88.43% <ø> (ø)
#lambda	92.33% <ø> (ø)

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[nl0]

The changes look good overall, but i think the huge rounded corners look a little out-of-place. i'd rather go for the standard border-radius, like this:

[fiskus]

the huge rounded corners look a little out-of-place. i'd rather go for the standard border-radius

Removed too much rounded corners as you propose

[nl0]

haven't tested it after the most recent changes, but i believe it should be ok