Install/uninstall/list plugins remotely

* replace npm and apt-get by yarn and apt in dockerfile.stretch

* keep consistancy

* Check pg_dump before backup

When the database is not on the same host pg_dump can't be not present

* Update upgrade.sh

Gentoo has not "stable" keyword for yarn (cf source, next line), you need to add "testing" keyword for yarn.
Source : https://packages.gentoo.org/packages/sys-apps/yarn

The media-libs/vips package does not exist on Gentoo (and all works fine on my PeerTube POC without vips).

* #1916 Load user by email - insensitive query

* Revert "Case insensitive login"

This reverts commit c1521ca3.

* #1916 Load user - insensitive query for username and sensitive for email

* #1916 Unit test for insensitive username login && documentation

Update postgresql version for Gentoo example

* #1915 Add a subject to contact form and add it to email sent

* #1915 Add subject to contact form tests

* #1915 Contact form test corrected && tslint correction