Fix crash on error detail page when using MariaDB

[phihos]

Instead of using {:array, :string} for the :breadcrumbs field in ErrorTracker.Occurrence we use a new custom field type ErrorTracker.Types.StringArray. It uses Jason to decode arrays when retrieving data from MySQL/MariaDB. Therefore it works very similarly to the :array type but without crashing on retrieval. This fix does not make a schema migration necessary. An additional test module ErrorTracker.StoreFetchTest was implemented to avoid regreessions in the future.

Fixes #150.

[phihos]

Ok, done refactoring now. Tests pass and code looks good to me at least.

[crbelaus]

Thanks for this contribution. I'm closing the PR because our contribution to ecto_sql has been accepted and included in version 3.13, so this is fixed with #161