[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2893: Refactor redundant `?t=json` handling code

[Tahoe-LAFS]

#2893: Refactor redundant `?t=json` handling code
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 Reporter:  exarkun            |          Owner:
     Type:  enhancement        |         Status:  new
 Priority:  normal             |      Milestone:  undecided
Component:  code-frontend-web  |        Version:  1.12.1
 Keywords:                     |  Launchpad Bug:
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 A number of web resources re-implement the logic to notice a request for
 json-format output and select a different renderer implementation.

 Refactor this to reduce duplication.  This refactoring will make the
 change to add a json format for the welcome page ([ticket:2476]) simpler.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2893>
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