Change the VariantType model - move publications to separate table

[dglemos]

Description

Improve the way we store variant types.
Create a separate table to store the publications linked to the variants.

Checks

• gene2phenotype/api/lgd/<g2p_id>/ - variant_type output has same structure
• update variant types
• updating same term: the inheritance is replaced with the new values (if empty then new value is empty); publications are appended (if not linked to variant yet)
• adding new term
• add new publication with variant types data
• create new record with variant types data
• delete variant type from record
• delete record that has variant type data
• gene2phenotype/api/activity_logs/
• merge records

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JIRA Ticket

G2P-473

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Contributor Checklist

• The code compiles and runs as expected
• Relevant unit tests are added or updated
• All unit tests are passing
• Code follows the G2P Coding Guidelines
• Documentation (code comments, confluence, etc.) has been updated as needed

[seeta-ramaraju]

LGTM. Changes have been tested.