Admin enhancements: new dashboard, reports, and error email alerts

New 'Logger Admin' dashboard + related reports + homepage

Closed #182 by adding several new metadata items:

• New 'Logger Admin' dashboard - this an initial attempt at providing a dashboard, but any feedback is welcome on how to further improve the dashboard
• New reports (used by the dashboard)
• New homepage for the Logger Console app that displays the 'Logger Admin' dashboard

New email alerts for Logger errors

Closed #229 by sending an email to the org's list of Apex Exception Email recipients (configured under Setup --> Email --> Apex Exception Email) any time an error occurs in Logger.

• This is being introduced primarily to help with troubleshooting any issues when saving LogEntryEvent__e records via the EventBus.publish() method, several other DML statements (for Log__c, LogEntry__c, etc.) have also been updated to send email alerts for any DML exceptions.
• Error email alerts can be enabled/disabled by updating the custom metadata type record LoggerParameter.SendErrorEmailNotifications (set to true by default)

New Slack plugin package v0.9.2-beta

Fixed #232 - the previous Slack plugin package was failing to install due to the custom metadata type LoggerSObjectHandlerPluginParameter__mdt being consolidated into LoggerParameter__mdt. Note: this plugin is still in beta - there are some upcoming plugin changes planned in #177 and within the plugin framework itself, so this plugin should still be considered a work-in-progress.

• Created new Slack plugin package v0.9.2-beta (based on v4.6.9)
• Updated the Slack README