network jobs: Refactor the redirect handling in AbdtractNetworkJob

Move the redirect handling from CheckServerJob to AbstractNetworkJob, so we can
later use it in any job  (in preparation for task #2614)
Settings dialog: remove the code that was updating the account icon

I just wanted to fix the warning:
 QObject::connect: No such slot OCC::SettingsDialog::slotUpdateAccountIcon(QIcon) in settingsdialog.cpp:108

It turns out we don't need anymore the signal or any of the code that
was updating the account icon in the settings dialog since it we now have a fixed icon
Connection Validator:  Fix connection warning

The timeout signal does not exist.
Timeout is handled as a normal network arror.
Download job: allow redirect on direct download link

Issue #2614
csync_update: do not force writing on the db for directory with a changed date

Fixes #2584
[tx-robot] updated from transifex
Download: When direct URL don't work, retry through webdav
Direct download URL: Fix resuming

The server might support resuming, so don't always erase the temporary file
and pass the startSize, so the temporary file will be remove if the server
does not support it after all (because it is not sending the "bytes" header

Also pass the expected etag for consistency even if it's not used in this case.