Render link to budget header

- Allow to define a link (text and url) on budget form for render on the budget
header.
- Improve styles

Co-authored-by: Senén Rodero Rodríguez <senenrodero@gmail.com>

app/controllers/admin/budgets_controller.rb

@@ -61,6 +61,7 @@ class Admin::BudgetsController < Admin::BaseController
       valid_attributes = [:phase,
                           :currency_symbol,
                           :voting_style,
+                          :main_link_url,
                           administrator_ids: [],
                           valuator_ids: []
       ] + descriptions