Share accounts: per-site Samba passwords from the webapp

Tools > Share accounts sets the pxe-upload, blancco and clonezilla Samba
passwords. Saving one runs smbpasswd and writes it everywhere it is used:
startnet.cmd in boot.wim (pxe-upload), the served Blancco preferences and
the live config.img (blancco), the iPXE menu's Clonezilla entries
(clonezilla). Refused while a PC is imaging unless forced. Stored 0600 in
UPDATES_DIR/shares/accounts.json, never shown back.

- The stored values are the authority: the playbook only creates the
  Samba accounts, seeds the store from site-vars (pxe_upload_password,
  blancco_smb_password, clonezilla_smb_password) for accounts it does not
  hold, and runs "share_accounts apply" at the end of every run.
- Every startnet save re-pins SHAREPASS, so a restored snapshot cannot
  bring an old password back; new menu blocks use %SHAREUSER%/%SHAREPASS%.
- Shop floor unattend Order 4 reads the login from fetch-source.txt;
  Invoke-PostPpkg deletes that file once the PPKG is applied.
- Upload-Image.ps1 asks for the password unless -Pass is given.
- pxe_admin_password: the Linux pxe login. Required while the box still
  has the shipped "pxe"; set with chpasswd when it differs.
- Blancco site settings now also reach the live config.img (BIOS path).

webapp/templates/base.html

@@ -78,6 +78,7 @@
         {{ navlink('Boot menu', url_for('startnet_editor'), ep == 'startnet_editor') }}
         {{ navlink('Enrollment packages', url_for('enrollment'), ep == 'enrollment') }}
         {{ navlink('Clonezilla backups', url_for('clonezilla_backups'), ep == 'clonezilla_backups') }}
+        {{ navlink('Share accounts', url_for('share_accounts.page'), ep.startswith('share_accounts.')) }}
         {{ navlink('Audit log', url_for('audit_log'), ep == 'audit_log') }}
       </div>
     </div>