Previously, public custom sections were only visible to logged-in users. In this PR, we are making them visible to anonymous as well.
The reason is that Community Section will be moved into custom section model to be easily editable by admins.
Allows users to configure their own custom sidebar sections with links withing Discourse instance. Links can be passed as relative path, for example "/tags" or full URL.
Only path is saved in DB, so when Discourse domain is changed, links will be still valid.
Feature is hidden behind SiteSetting.enable_custom_sidebar_sections. This hidden setting determines the group which members have access to this new feature.
