Add default value for Application.post_logout_redirect_uris

[tonial]

Fixes #1309

Description of the Change

Adding a default value to post_logout_redirect_uris model and migration

Checklist

• [x] PR only contains one change (considered splitting up PR)
• [ ] unit-test added
• [ ] documentation updated
• [ ] CHANGELOG.md updated (only for user relevant changes)
• [x] author name in AUTHORS

[codecov[bot]]

Codecov Report

Merging #1319 (5772e2f) into master (6bca431) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1319   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files          32       32           
  Lines        2073     2073           
=======================================
  Hits         2020     2020           
  Misses         53       53           

Files Changed	Coverage Δ
oauth2_provider/models.py	98.57% <ø> (ø)

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