Nav menu plus menu_links() hook

[simonw]

Closes #1064, refs #690.

[codecov]

Codecov Report

Merging #1065 into main will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1065      +/-   ##
==========================================
+ Coverage   91.23%   91.27%   +0.03%     
==========================================
  Files          28       29       +1     
  Lines        3710     3724      +14     
==========================================
+ Hits         3385     3399      +14     
  Misses        325      325              

Impacted Files	Coverage Δ
datasette/plugins.py	82.35% <ø> (ø)
datasette/app.py	96.42% <100.00%> (+0.03%)	⬆️
datasette/default_menu_links.py	100.00% <100.00%> (ø)
datasette/hookspecs.py	100.00% <100.00%> (ø)

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