Fix broken links in some Instance-level integrations

When the feature flag instance_level_integrations is enabled, some integrations on Admin > Settings > Integrations return an error page. This is because they have custom templates that include some references / links to project. They should be updated to be re-usable across project / group / admin. The integrations affected are:

• /admin/application_settings/integrations/alerts/edit
• /admin/application_settings/integrations/mattermost_slash_commands/edit
• /admin/application_settings/integrations/gitlab_slack_application/edit
• /admin/application_settings/integrations/slack_slash_commands/edit

Note: This issue exists in Service Templates as well.