add elasticsearch support, add --depth to git clone commands + other improvments

[eshaan7]

• Add elasticsearch support. Closes Elastic Search Support #143.
• I have moved docs for Optional analyzers, Elastic Search configuration, and Django Permissions & Group to an additional file called Advanced-Usage.md in the documentation.
• Added --depth 1 flag to git clone commands with this it only clones the latest revision so speeding up build time and uses less storage.
• example kibana config added under configuration/
• Documentation related to elastic search.
• Fixed job.file.path being empty issue during Job serialization.

In models.py,

• I increased the max_length of observable_name column to 512. For Issue Integration/connectors for other threat intel projects #144.
• Set max_length for md5 column to 32 because md5 hashes have a fixed length of 32.
• .. few other max_length changes I felt were appropriate.