Consolidate implementation of right scrollbars for issues, merge requests, milestones, and epics

Description

We currently have various implementations of the collapsible right sidebar in use in the following objects:

• issues
• merge requests
• milestones
• epics

They're currently either implemented as:

• Ruby/HAML templates
• Hybrid of Ruby/HAML with varying degrees of JavaScript/Vue component mounts and functionality enhancements
• Complete Vue-based components (Epics in EE)

Proposal

Consolidate the use of these sidebars into (ideally) a single implementation so that changes, enhancements, and maintenance is simplified, e.g., adding tooltips.

Links / references

• gitlab-ce#25010, namely the amount of work required to add tooltips to sidebars 😄