[AQTS-998] Work days SLA analytics columns for each application to support with working days consideration

[Hassanmir92]

Ticket: https://dfedigital.atlassian.net/browse/AQTS-998

1. Introduce a new "verification_started_at" timestamp which will be used to support calculate when an assessment of a specific application moved from initial assessment into verification stage. This timestamp will be set when application first moves into the "verification" stage. Considerations have been made for when a decline decision has been reversed, this timestamp is only ever set once.
2. Updating the England holiday dates for 2027.
3. Renaming, removing and adding columns related to the working days stats for application_forms, assessments and further_information_requests. These columns are used to run reports for SLA's within BigQuery where the concept of working days is not possible. This is something that our system had already been doing but the columns and their purpose are now very much out of date as the system had developed over the last 2 years. This commit adds any new working days columns required, removes columns any that are no longer used and bring no value and renaming columns to ensure there's a consistency in the pattern of naming them and making it very clear for data analysts on what they represent.
4. Updating the UpdateWorkingDaysJob class to consider the new column changes made in this PR.

Columns are:

# Within application_forms table
working_days_between_submitted_and_today
working_days_between_submitted_and_completed

# Within assessments table
working_days_between_started_and_today
working_days_between_started_and_completed
working_days_between_submitted_and_started
working_days_between_started_and_verification_started
working_days_between_submitted_and_verification_started

# Within further_information_requests table
working_days_between_assessment_started_to_requested

Release process:

1. Merge this PR.
2. Merge the dataform PR

[github-actions]

Review app deployed to https://apply-for-qts-review-2767-web.test.teacherservices.cloud/personas

[syed87]

lgtm

[github-actions]

Review app for PR 2767 was deleted