Fix a few issues reported by PCP (Plugin Check Plugin)

[jorgeatorres]

This PR fixes a few violations reported by the Plugin Check Plugin (PCP) from wporg's plugin team. In particular it addresses issues issues in all categories from PCP except for "Security" (which I'm still looking into).

The aim here is to improve compliance with the wporg review process, not to fix all issues with our codebase (of which there are still quite a few).

Related to #1027.

Testing instructions

1. Make sure things are working ok. IMHO it should be enough to make sure tests pass, but some smoke testing wouldn't hurt.
2. To make sure issues reported by PCP are fixed:
   1. Install "Plugin Check Plugin" in Plugins > Add New Plugin.
   2. Go to Tools > Plugin Check.
   3. Choose "Action Scheduler".
   4. Check off all categories except for "Security".
   5. Click "Check it!".
   6. Make sure violations reported are either in tests or false positives.