Fixes https://github.com/owncloud/core/issues/13580

Moved external share JS code into a small plugin to make it testable.

Added unit test for the external share dialog logic + ajax calls.

* fixes #13460

There are a lot of clients that support multiple WebDAV accounts in the same application. However, they resent all the cookies they received from one of the accounts also to the other one. In the case of ownCloud this means that we will always show the user from the session and not the user that is specified in the basic authentication header.

This patch adds a workaround the following way:

1. If the user authenticates via the Sabre Auth Connector add a hint to the session that this was authorized via Basic Auth (this is to prevent logout CSRF)
2. If the request contains this hint and the username specified in the basic auth header differs from the one in the session relogin the user using basic auth

Fixes https://github.com/owncloud/core/issues/11400 and https://github.com/owncloud/core/issues/13245 and probably some other issues as well.

This requires proper testing also considering LDAP / Shibboleth and whatever instances.

minimum browser window width ≈ 1280px

as expected with inline-block span

In file lists with no selection checkbox, the CSS selector is different.
The thumbnail also needs position:absolute and a z-index to be displayed
above the row's grey highlight.

on src change

`$dir` may for example very well not get passed at well.