Run gulp pipeline as part of the CI

[sebastienros]

Run the gulp pipeline and check the result is the same as the checked-in file to ensure the PR correctly ran it if there were changes in js/css.

c.f. https://github.com/OrchardCMS/OrchardCore/pull/11893/files

[0liver]

This should be addressed soon, because with a fresh clone of OC, using Node.js 16 + NPM 8, I get 7 modified files compared to what's committed to dev:

The changes don't seem substantial, it's only code being moved around, but they are still changes that should be committed.

When using Node.js 14 with NPM 6, the Gulp build actually fails due to NPM dependencies not automatically being installed and Orchard.Setup choking on that because of the missing @popperjs dependency of bootstrap.

[Piedone]

This was fixed in #15735.