Add models and tables for tags and timeline_event_tag links

What does this MR do and why?

Adds database table and model along with specs for MVC from #373853 (closed)

Database migrations

UP

bundle exec rake db:migrate
main: == 20221005072353 CreateIncidentManagementTimelineEventTags: migrating ========
main: -- create_table(:incident_management_timeline_event_tags, {})
main:    -> 0.0182s
main: == 20221005072353 CreateIncidentManagementTimelineEventTags: migrated (0.0189s)

main: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: migrating ====
main: -- create_table(:incident_management_timeline_event_tag_links, {})
main:    -> 0.0021s
main: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: migrated (0.0022s)

ci: == 20221005072353 CreateIncidentManagementTimelineEventTags: migrating ========
ci: -- create_table(:incident_management_timeline_event_tags, {})
ci:    -> 0.0122s
ci: == 20221005072353 CreateIncidentManagementTimelineEventTags: migrated (0.0123s)

ci: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: migrating ====
ci: -- create_table(:incident_management_timeline_event_tag_links, {})
ci:    -> 0.0026s
ci: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: migrated (0.0027s)

DOWN

bundle exec rake db:migrate:down:main VERSION=20221005094926
main: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: reverting ====
main: -- drop_table(:incident_management_timeline_event_tag_links)
main:    -> 0.0028s
main: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: reverted (0.0031s)

bundle exec rake db:migrate:down:ci VERSION=20221005094926
ci: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: reverting ====
ci: -- drop_table(:incident_management_timeline_event_tag_links)
ci:    -> 0.0038s
ci: == 20221005094926 CreateIncidentManagementTimelineEventTagLinks: reverted (0.0045s)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373853 (closed)