server: add configuration for install notifications

[matthewpi]

Adds two new config options for controlling when install notifications should be sent to a server owner (PTERODACTYL_SEND_INSTALL_NOTIFICATION and PTERODACTYL_SEND_REINSTALL_NOTIFICATION, both default to true). This PR also adds a new installed_at field to servers which tracks the time a server was last "installed", this is primarily used to know if a server is being reinstalled and is not currently mapped in any API responses; this column defaults to null rather than setting the current date.