Unify background colour for "create" buttons in admin panel

[bertocq]

Thanks to @aitbw at #1415 we noticed that "Create" buttons in the admin panels use blue and green background without any pattern/reason behind, creating inconsistency.

Background

Right now:

• Blue buttons /admin/budgets, /admin/newsletters and /admin/admin_notifications
• Green buttons /admin/banners /admin/geozones /admin/tags /admin/legislation/processes

What we need

Adds consistency on admin buttons colors following this styles:

[aitbw]

I tend to associate the green color with life, creation so I think, UX-wise, it should be the default color for buttons when you want to create a new record —blue seems more appropriate for when you want to offer the user in-depth information about something

[decabeza]

Hi @bertocq and @aitbw! Thanks for the issue! 🙏 Definitely consistency is the key! 😌

I updated the issue description with a "mini-guide" for buttons on Admin.

[bertocq]

Awesome @decabeza thats a great reference! Should that go into docs repo? 🤔 don't want to forget/loose it :)