As per https://gitlab.com/gitlab-org/gitlab-ce/issues/46043, project
templates should be squashed before updating, so that repositories
created from these templates don't include the full history of the
backing repository.

This rake task had been broken for a while. This fixes the breakages,
adds a test to help avoid future breakages, and adds a few ergonomic
improvements to the task itself.

Merge branch '57794-project-template-for-net' of https://gitlab.com/gitlab-org/gitlab-ce into 57794-project-template-for-net

Merge branch '57785-create-project-template-for-netlify' of https://gitlab.com/gitlab-org/gitlab-ce into 57785-create-project-template-for-netlify

Use gitlab-svg to provide the icons for rails, spring and express. For
the new GitLab pages templates, we are falling back to the GitLab logo.

Adds templates for the 5 most popular Pages templates to the new project
menu. This does not add unique icons for the templates because that
turned out more complicated than expected, and this feature is valuable
without them.

Related to !13108. Mostly this is just running the rake task and
changing the task a bit to catch cases like the project already existing
or so. The rake task moves archives to the vendor/project_template
directory, which are checked in too.

First round of review, main changes:
- templates.title is human readable, #name will be passed around
- GitLabProjectImporterService has been renamed

First iteration, and some stuff is missing. But basically this rake task
does a clone of a project we've pointed it to. Than creates a project on
the GDK, which should be running in the background. This project is
exported, after which we move that archive to the location we need it.
We clean up by removing the generated project.

The first idea was to export the project on .com too, however than we
might run into ImportExport versions mismatch. This could've been
circumvented by checkout out an older commit locally. This however is
not needed yet, so we opted to not go this route yet, instead we will
iterate on what we got.

Started implementation for the first iteration of
gitlab-org/gitlab-ce#32420. This will allow users to select a template
to start with, instead of an empty repository in the project just
created.

Internally this is basically a small extension of the ImportExport
GitLab projects we already support. We just import a certain import
tar archive. This commits includes the first one: Ruby on Rails. In the
future more will be added.