Create SearchLogger model

[JPrevost]

Why are these changes being introduced:

• This will simplify logic in GraphQL while making it easier to expand the logic necessary to add more complex logic to preprocess terms for each source or do any other normalization to incoming data.

Relevant ticket(s):

• Pre-work for: https://mitlibraries.atlassian.net/browse/TCO-113

Summary of changes (please refer to commit messages for full details)

Developer

Ticket(s)

https://mitlibraries.atlassian.net/browse/TCO-###

Accessibility

• ANDI or Wave has been run in accordance to our guide and
  all issues introduced by these changes have been resolved or opened
  as new issues (link to those issues in the Pull Request details above)
• There are no accessibility implications to this change

Documentation

• Project documentation has been updated, and yard output previewed
• No documentation changes are needed

ENV

• All new ENV is documented in README.
• All new ENV has been added to Heroku Pipeline, Staging and Prod.
• ENV has not changed.

Stakeholders

• Stakeholder approval has been confirmed
• Stakeholder approval is not needed

Dependencies and migrations

NO dependencies are updated

NO migrations are included

Reviewer

Code

• I have confirmed that the code works as intended.
• Any CodeClimate issues have been fixed or confirmed as
  added technical debt.

Documentation

• The commit message is clear and follows our guidelines
  (not just this pull request message).
• The documentation has been updated or is unnecessary.
• New dependencies are appropriate or there were no changes.

Testing

• There are appropriate tests covering any new functionality.
• No additional test coverage is required.

[matt-bernhardt]

This seems pretty straightforward. I'm not quite sure whether the pre-processor will slide into the log_search_event method or into the SearchLogger method as a whole, but I'm not sure I need to know that now. For this PR, this change looks fine.

[JPrevost]

@matt-bernhardt I'm intending to leave the graphql side of things super clean. It just asks to log a term. The SearchLogger will coordinate any necessary preprocessing to get the data in shape for logging the Term/SearchEvent.