Add gravcms module

[mission-deny-the-mission]

Module has not been confirmed exploitable yet. Problem with opening shell after running metasploit exploit. Please advise

[cliffe]

Possibly working, some exploitation complications? Note: needs testing.

[JD2344]

Did some further tests re this module. Seems to be vulnerable to the basic POC, however could not get the Metasploit module to open the shell. Ill take another look and see if i can manually invoke the endpoint that the exploit utilises and see if there is an issue with the basic setup or not.