Add a button to display all issues with no labels

[finnhodgkin]

Adds:

• an extra seed script that gets all issues with no labels
• a new db table for issues with no labels
• github hooks for issues with no labels
• a route for label-less issues
• tests

The code isn't very dry - the two index.html.eexs are practically identical and there's a lot of repeating stuff in hook_controller. Should we be pulling more stuff out to functions / using child templates?

fixes #229

[codecov]

Codecov Report

Merging #236 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #236   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      8    +1     
  Lines          82    127   +45     
=====================================
+ Hits           82    127   +45

Impacted Files	Coverage Δ
web/controllers/page_controller.ex	100% <100%> (ø)	⬆️
web/github/github_api.ex	100% <100%> (ø)	⬆️
web/controllers/unlabelled_controller.ex	100% <100%> (ø)
web/controllers/hook_controller.ex	100% <100%> (ø)	⬆️
test/support/test_helpers.ex	100% <100%> (ø)	⬆️

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[samhstn]

Why do we need to add another table?

Why would the issues not suffice, we have a labels column?