syncthing: Create LDAP group name different from system group

When installing the syncthing app, create a LDAP group named "syncthing-access" instead of "syncthing", because the app creates the "syncthing" system group to run the syncthing daemon. Duplicate group names can cause some ambiguity as described in #2008 (closed).

• Rename the existing "syncthing" LDAP and Django group to "syncthing-access".
• Update existing web shares to be accessible with new group name.
• Functional tests: Add tests to check access to the syncthing site with a user in the syncthing-access group and no group.
• Functional tests: Scroll to the edit button before clicking. Fixes some test failures on a smaller browser window.

Fixes #2008 (closed)

Tests performed on Debian stable and testing:

• Check that the existing "syncthing" group is renamed after upgrade:

1. Without patch applied, install syncthing, create a user in group "syncthing".
2. Apply patch, update Apache2 config file /etc/apache2/conf-available/syncthing-plinth.conf, reload Apache2, restart plinth.
3. Check that the created user is now in the "syncthing-access" group and can access /syncthing site.

• Check that the app upgrade succeeds when there are no users in the syncthing group.

• Create a web share accessible by the 'syncthing' group. Check that after the upgrade, the share is accessible to a member of syncthing-access group.

• All the syncthing app tests pass.