Implemented statistics dashboard

[thejohnbrown]

Hi,

This PR introduces a dashboard page which provides charts and lists for visualising vulnerability and engagement statistics in Writehat based on a specified start and end date.

The current charts/lists are:

• Total engagements
• Engagements started by name
• Vulnerabilities by severity (different patterns are used to distinguish between CVSS, DREAD, etc. as can be seen in the below screenshot)
• Vulnerabilities by category
• Vulnerabilities by Requestor/Customer

Let me know what you think and if any changes need making before merging.

[thejohnbrown]

Is there anything else required to get this merged?