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.
The latter can be called directly from the Topic page object,
so we can remove some duplication between the two. There are
levels of page objects (e.g. entire page, component, complete flow)
and its perfectly valid to call one from another.
