Fix notify_only_default_branch check for Slack service

The notify_only_default_branch property is using boolean_accessor
this means we need to check it using a question methods.
Also add specs for disabling this option.
49 jobs for mrchrisw-fix-slack-notify in 106 minutes and 22 seconds (queued for 1 minute)
Status Job ID Name Coverage
  Build
manual #15797068
allowed to fail manual
build-package
 
  Prepare
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knapsack

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  Test
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docs lint

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downtime_check

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rake mysql db:seed_fu

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rake pg db:migrate:reset

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rake pg db:rollback

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rake pg db:seed_fu

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static-analysis

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  Post Test
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coverage

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lint:javascript:report

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