Add unit test for selection overlay actions in trashbin

Add a test for ObjectStore\NoopScanner

adding a windows smb config for testing to ci

Avatars in share dialog fixes

Since aa821ecc leading and trailing slashes are
trimmed in the file cache paths

Update scanFile() and scanChildren() to the new signature of the parent ...

Remove unnecessary backslashes from translations

Let users configure security headers in their Webserver

* Avatar for "xxxx share with you..." to the left
* Avatars for groups and remote shares (use default placeholder)
* Modified and added unit tests
* Use the same css for all the avatars in the dropdown

Jenkins #14650

We were using the SMB and SFTP backends, which since Icewind's new SMB library
results in an actual attempted connection just by constructing the storage.
This commit replaces all references to real storages (except when necessary)
with a dummy storage class.

use `dbtableprefix` for temp table and index names

Read from IRequest instead of reading twice

Add a repair step to delete old tables

proper description of appcodechecker in config sample

[Master] Remove hacky Substring support for MSSQL

fix for loading spinner reappearing after errors in db-connection

Remove "Download from URL" feature

Allow configuring background job mode from the console

Fixes https://github.com/owncloud/core/issues/13326

Doing this in the PHP code is not the right approach for multiple reasons:

1. A bug in the PHP code prevents them from being added to the response.
2. They are only added when something is served via PHP and not in other cases (that makes for example the newest IE UXSS which is not yet patched by Microsoft exploitable on ownCloud)
3. Some headers such as the Strict-Transport-Security might require custom modifications by administrators. This was not possible before and lead to buggy situations.

This pull request moves those headers out of the PHP code and adds a security check to the admin settings performed via JS.

Maintenance mode message might be misleading

Disable the cache updater when doing the encryption migration

Cleanup config.sample.php