Adjust TTM and PR size metrics

[gonzaM]

What does this PR do?

JIRA tickets:

• https://rootstrap.atlassian.net/browse/EM-372
• https://rootstrap.atlassian.net/browse/EM-373
• https://rootstrap.atlassian.net/browse/EM-383

Description

This PR introduces two improvements to the metrics:

Average Calculation:

• Add average calculation.
• Update UI to show average.

Ignored Users:

• Add new scopes in User model: bot_users and ignored_users
• Implement SettingsService support for configurable ignored users
• Exclude ignored users from metric calculations
• Update UI to indicate bot exclusion in metrics

Note:

• The Code Climate section was removed from the UI since it is not used at the moment.
• The Review turnaround section was removed from the UI since it is not used. Pending to remove more code associated.