Add a job for Vanir signature generation

[cuixq]

We are planning to migrate worker to Go; however Vanir is written in Python so we probably need a separate job to generate Vanir signature which we can keep the implementation in Python.

The plan is to add a separate job to generate Vanir signatures:

• We are going to store the timestamp where the last job was run
• Vanir signature generation will be run against GIT vulnerabilities modified since the last timestamp

We may want to optimize the performance by using gitter for git cloning, which requires a bit work on Vanir's code extractor.