Address inconsistent names in partitioning helpers

What does this MR do?

In code from the initial iterations of the partitioning migration helpers, I noticed that the same tables were referred to with multiple different names, i.e. the original table was sometimes referred to as table, source_table or template_table.

This is a very light refactoring MR to use consistent naming for both the source table and the partitioned table throughout the migration helper. Also addressed a few other minor issues, like abbreviated names and some inconsistent ordering of method parameters.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
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• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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