[Backport] Admin permissions in legislation proposals

[javierm]

References

• Commit AyuntamientoMadrid/consul@01ce8c9
• Commit AyuntamientoMadrid/consul@4a0ac88
• Pull request Create proposal redirect AyuntamientoMadrid/consul#983

Objectives

• Ease the backport of Allow select winner legislation proposals AyuntamientoMadrid/consul#1640 and Simplify custom legislation proposals AyuntamientoMadrid/consul#1651, since they're related to the actions in legislation proposals.
• Reduce the differences between CONSUL's repository and Decide Madrid's repository.

Notes

• ⚠️ Maybe none of the commits here should be backported.
• Create proposal redirect AyuntamientoMadrid/consul#983 isn't being backported completely because the code affecting the link to new proposal has been changed in Simplify custom legislation proposals AyuntamientoMadrid/consul#1651, which will be backported soon.

[javierm]

Travis failure is not related to this pull request.