Compute ACS Url in IdP form

[augustuswm]

When creating a SAML identity provider connection, the ACS URL that the user enters should always be of the form:

http(s)://<silo>.sys.<subdomain>/login/<silo>/saml/<idp>

where <silo> is the Silo name, <subdomain> is the subdomain assigned to the rack, and <idp> is the name of the IdP connection. This means that when a user is filling out the form, we have all of the necessary information to compute this value once they enter a value in to the name field.

By computing this we can also make the field readonly to prevent accidental changes.