refactor: sidebar data providers can be refreshed separatelly

We've got two views in the sidebar: "Issues and Merge Requests" (issuable) and "For current branch" (current_branch). They are refreshed with the gl.refreshSidebar command but we will need to refresh them separately because the current_branch view is going to be refreshed at regular intervals.

This MR refactors the refreshing logic that hasn't been updated since we adopted the extension over a year ago.

Related to #268 (closed)