feat: add scope control for password complexity rules

[rohasmsk]

Summary

• Add two checkboxes to the "Minimum password requirements" section in
  admin security settings: Apply to user account passwords and
  Apply to public link passwords (both enabled by default —
  no change in behavior for existing installations)
• Complexity rules (min length, lowercase, uppercase, numbers, special
  chars) are now skipped for the unchecked scope; password history,
  expiration and force-change-on-first-login rules are unaffected and
  continue to apply to user accounts only

Motivation

Workaround for #359
This change provides a workaround for #359.

Installations using OpenID Connect (or other external identity
providers) can now disable complexity checks for user account passwords,
since those users authenticate via their IdP which enforces its own
password policies. Applying ownCloud's complexity rules on top is
redundant and causes friction. At the same time, complexity rules for
public link passwords remain fully functional.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Maksim Martynov seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}