Configurable special login route for admins

**Is your feature request related to a problem? Please describe.**

When you only activate the external account to login on your org (sso) admins have no way of getting a login page and do not always want have access on the external account system.

**Describe the solution you'd like**

We implemented a custom route for admins /admin_sign_in for this purpose. We propose to add a config flag on the organization edit form to activate or deactivate it.

**Describe alternatives you've considered**

Not even bother to add a config flag in /system and just add it by default on Decidim.

**Does this issue could impact on users private data?**

No

**Funded by**

Open Source Politics