Add a feature setting for proposals

[voodoorai2000]

What

Allow admins to deactivate the proposal's section

How

• Adding a setting for proposals[1]
• Using a feature_flag[2] in the proposals_controller
• Using the setting to decide if recommended proposals should be displayed in the home page[3]

[1] https://github.com/consul/consul/blob/master/db/seeds.rb#L72
[2] https://github.com/consul/consul/blob/master/app/controllers/debates_controller.rb#L9
[3] #1824

[aitbw]

I'll handle this one.