ref(releases): Move release models into releases module

[cmanallen]

Part one of moving release code into a releases module. See the plan here.

For now we move all the models into the releases module and re-export those models through their pre-existing file locations. This keeps the change minimally disruptive and allows us to iteratively change the structure of the releases code.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
26855	1	26854	205

View the top 1 failed test(s) by shortest run time

tests.sentry.models.releases.test_release_project.ReleaseProjectManagerTestCase::test_post_save_signal_runs_if_dynamic_sampling_is_enabled_and_latest_release_rule_exists

Stack Traces | 15.6s run time

#x1B[1m#x1B[.../models/releases/test_release_project.py#x1B[0m:65: in test_post_save_signal_runs_if_dynamic_sampling_is_enabled_and_latest_release_rule_exists
    assert mock_task.mock_calls == [
#x1B[1m#x1B[31mE   AssertionError: assert [] == [call(project...t.post_save')]#x1B[0m
#x1B[1m#x1B[31mE     #x1B[0m
#x1B[1m#x1B[31mE     Right contains one more item: call(project_id=4556439601676290, trigger='releaseproject.post_save')#x1B[0m
#x1B[1m#x1B[31mE     #x1B[0m
#x1B[1m#x1B[31mE     Full diff:#x1B[0m
#x1B[1m#x1B[31mE     + []#x1B[0m
#x1B[1m#x1B[31mE     - [#x1B[0m
#x1B[1m#x1B[31mE     -     call(project_id=4556439601676290, trigger='releaseproject.post_save'),#x1B[0m
#x1B[1m#x1B[31mE     - ]#x1B[0m

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