Use a constant to store the menu order
Tweak the names of the menu items

Fix #73
Add sidebar to system info page
Add information about `DISABLE_WP_CRON` in system info
Tweak labels in Scheduled Jobs page

Fix #71
Fix a notice that happened because of duplicate call to `add_meta_boxes`
Removed unused variables
Remove old compatibility code for `The Events Calendar` plugin

This code was added for #14. But the plugin has already made the
necessary changes and the compatibility code is no longer needed.
Add filters to extend menu items

Fix #74
Add the ability to delete Jetpack contact form messages

Fix #72