Add an indefinite loading bar on the top of pages with deferred rendering

Some pages have a lot of embedded information and have to execute a lot of instructions due to which the rendering can be deferred. An indefinite loading bar at the top of the page would let the user know about that so that they do not have to worry about the actions being done in the background or not.

Start by locating the frontend entry point where deferred rendering begins and the styles used for the page header. Check how rendering completion can be detected, then confirm the top loading bar remains visible during deferred work and disappears when it finishes.

Written by the indexing model from the issue text.