Adds optional logging setting that logs send attempts

[pgraham3]

• Adds Enable Logs option to plugin settings (currently enabled by default when the plugin is initially activated)
• If enabled, logs are stored in a db table and a Logs tab is available in the settings UI
• Logs tab is hidden if setting is not enabled
• Each log contains the datetime, from address, to address, subject, and Postmark API response
• Load More button appears in Logs tab if there are more logs to load. 10 are loaded at a time.
• Cron job runs once per day to check for and remove logs older than 7 days
• Logs table is removed on plugin deactivation or settings change to no longer log (good citizen stuff 👍)
• Small cleanup of JS/CSS inclusion to use WordPress standards of register and enqueue