Use _get_resource in App.slug

[mbirtwell]

We should expect a single slug here because we're providing an id

[codecov-commenter]

Codecov Report

Base: 48.27% // Head: 50.00% // Increases project coverage by +1.72% 🎉

Coverage data is based on head (7819131) compared to base (9d495eb).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   48.27%   50.00%   +1.72%     
==========================================
  Files          30       32       +2     
  Lines        1307     1436     +129     
  Branches      164      178      +14     
==========================================
+ Hits          631      718      +87     
- Misses        662      703      +41     
- Partials       14       15       +1     

Impacted Files	Coverage Δ
heroku3/models/app.py	32.12% <0.00%> (ø)
heroku3/models/__init__.py	65.57% <0.00%> (ø)
heroku3/__init__.py	100.00% <0.00%> (ø)

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[martyzz1]

fair point. thanks