[Upstream] Improve Admin settings section

[decabeza]

References

Upstream from consuldemocracy#3328.

Objectives

This PR clean unused settings and improve texts to explain better the settings:

• Remove unused settings "blog_url", "transparency_url" and "opendata_url": these settings are customised for Madrid's fork. On CONSUL any user can include new links using site customisation content blocks from admin panel.
• Remove proposal improvement path setting.
• Remove unused place name setting.
• Improve i18n for admin settings explaining with better descriptions.
• Add missing verification settings to dev seeds.
• Remove legacy banner images and banner styles settings: also removed all content related with this feature.
• Fix hound warnings on dev_seeds.