Fix saving inline alerts

[pereirinha]

Fixes #1527.

The issue has 2 parts.

1. Saving inline the Alert will fire the Admin AJAX inline_save action. In its internals, it expected information about the date and time of the post to be available as POST arguments. We are adding this information as hidden fields;
2. There are cases where we are not saving a context or a connector. We should ensure a fallback;

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Ensure that WordPress's core inline_save action has all the information it needs to save the Stream Alert post type.
• Fix: Handle partial data for Connector or Context

[marcinkrzeminski]

@pereirinha LGTM 🚀. I have tested this locally and can confirm the issue is gone. Related tests also are 🟢.