Add an option to disable search for homeservers which may not be interested in it

[turt2live]

This is useful for homeservers not intended for users, such as bot-only homeservers or ones that only process IoT data.

This was done with a community hat on:
Signed-off-by: Travis Ralston [email protected]

[codecov-io]

Codecov Report

Merging #4230 into develop will increase coverage by 6.87%.
The diff coverage is 14.28%.

@@             Coverage Diff             @@
##           develop    #4230      +/-   ##
===========================================
+ Coverage    66.65%   73.52%   +6.87%     
===========================================
  Files          299      299              
  Lines        29796    29813      +17     
  Branches      4871     4875       +4     
===========================================
+ Hits         19861    21921    +2060     
+ Misses        8501     6460    -2041     
+ Partials      1434     1432       -2

Impacted Files	Coverage Δ
synapse/storage/search.py	62.03% <0%> (-0.95%)	⬇️
synapse/handlers/search.py	80.24% <0%> (+2.74%)	⬆️
synapse/config/server.py	63.88% <100%> (+6.87%)	⬆️
synapse/handlers/federation.py	61.72% <0%> (+0.1%)	⬆️
synapse/handlers/user_directory.py	70.73% <0%> (+0.3%)	⬆️
synapse/handlers/sync.py	92.05% <0%> (+0.44%)	⬆️
synapse/rest/client/v1/room.py	92.75% <0%> (+0.5%)	⬆️
synapse/rest/client/v1/presence.py	65.95% <0%> (+1.06%)	⬆️
synapse/push/httppusher.py	68.23% <0%> (+1.17%)	⬆️
synapse/handlers/directory.py	73.12% <0%> (+1.25%)	⬆️
... and 111 more

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[richvdh]

lgtm