Add `project_settings` to high-traffic tables list

What does this MR do?

Add project_settings to high-traffic tables list

As of today, we have around 14 million records, the size of the table still acceptable (around 781MB), but this record is created whatever a new project is created so this table can expect a high-organic traffic in the future.

gitlabhq_production=> select pg_size_pretty(pg_relation_size('project_settings'));
 pg_size_pretty
----------------
 781 MB
(1 row)

gitlabhq_production=> select count(*) from project_settings;
  count
----------
 14440349
(1 row)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] 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.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done