As the infra team we decided to have an hard timeout of 3 hours on all
the jobs: while this sometimes causes spurious timeout errors it's a
great limit to prevent CI time to grow even more.

Co-Authored-By: Jake Goulding <shepmaster@mac.com>

That was just used because Travis doesn't have time stamps on all log
lines, but Azure does, so no need to add our own.

This commit is intended to go through and review/refactor the azure
pipelines configuration we have. The major changes are:

* The separate `{windows,macos,linux}.yml` files are now all merged into
  one `run.yml`. This allows a shared "master flow" for all platforms
  with divergence only where necessary.

* Some install steps have been separated as `install-*.yml` scripts,
  where each script internally matches on the appropriate OS and then
  delegates accordingly.

* Some various bits and pieces of cruft have been removed which were
  artifacts of Travis's setup or similar.

Make sure there's one dist and one test builder for each of the three
main platforms

This takes 30+ seconds to check out on Windows, so let's speed it up
through a tarball like we do other big git repositories.

Make sure `succeeded()` is in all the conditionals

Some review feedback and other misc tweaks

Let's see how long a full build takes.

The one locked here has a dependency on an old enough `cmake` crate that
it's not detecting visual studio correctly. Let's update webrender which
updates `cmake` which should be able to detect Visual Studio correctly.

* Clean up the `install-windows-build-deps.yml` file and add some more
  comments where appropriate.

* Add some comments to `run.yml`

* Don't fast path the `rustfmt` submodule, but we'll take care of that
  later if necessary.