Only subscribe for active subscriptions

[joshmn]

Campaign#subscribe should only find_or_create for subscriptions that haven't ended.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (925b939) 98.62% compared to head (9e25105) 98.63%.

:exclamation: Current head 9e25105 differs from pull request most recent head c1dbb4d. Consider uploading reports for the commit c1dbb4d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31 +/- ## ======================================= Coverage 98.62% 98.63% ======================================= Files 98 98 Lines 2479 2497 +18 ======================================= + Hits 2445 2463 +18 Misses 34 34 ``` | [Impacted Files](https://codecov.io/gh/joshmn/caffeinate/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [app/models/caffeinate/campaign.rb](https://codecov.io/gh/joshmn/caffeinate/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YXBwL21vZGVscy9jYWZmZWluYXRlL2NhbXBhaWduLnJi) | `100.00% <100.00%> (ø)` | | | [...ec/models/caffeinate/campaign\_subscription\_spec.rb](https://codecov.io/gh/joshmn/caffeinate/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3BlYy9tb2RlbHMvY2FmZmVpbmF0ZS9jYW1wYWlnbl9zdWJzY3JpcHRpb25fc3BlYy5yYg==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[jon-sully]

🙌