Show Ajax requests in performance bar

Closes #43925

See merge request gitlab-org/gitlab-ce!17742

But first, rewrite the performance bar in Vue:

1. Remove the peek-host gem and replace it with existing code. This also allows
   us to include the host in the JSON response, rather than in the page HTML.
2. Leave the line profiler parts as here-be-dragons: nicer would be a separate
   endpoint for these, so we could use them on Ajax requests too.
3. The performance bar is too fiddly to rewrite right now, so apply the same
   logic to that.

Then, add features! All requests made through Axios are able to be tracked. To
keep a lid on memory usage, only the first two requests for a given URL are
tracked, though. Each request that's tracked has the same data as the initial
page load, with the exception of the performance bar and the line profiler, as
explained above.

'/-/' delimiter is used only in UI, in API we don't use it for
other endpoints. To align search endpoints with the rest of API
endpoints, this patch makes '/-/' optional for existing endpoints (to
keep backward compatibility).

Documentation is updated to prefer paths without '/-/'.

Resolve "Viewing a particular commit gives 500 error ~ Error (undefined method `binary?')"

Closes #44257

See merge request gitlab-org/gitlab-ce!17805

 Conflicts:
   Gemfile.lock

Closes #44365

These can be a `BatchLoader` which is proxying a nil, while not being concrete
nils themselves.

Resolve "Extend API for exporting a project with option to overwrite project description"

Closes #41902 and #34936

See merge request gitlab-org/gitlab-ce!17744

Eliminate Faraday warnings

Closes #44126

See merge request gitlab-org/gitlab-ce!17795