Make used cached credentials a global setting

Users in a corporate environment are required to change their passwords quite often, which leads to locked accounts if Continuum has stored the old password. Besides that, storing passwords in plain text as Continuum does violates the security rules.

Therefore, we train everyone to check the "use cached credentials" box when adding projects, and run a daily cron job with an sql statement to modify the database (set the flag, delete userid/password) for those who forget.

Since we want Continuum to NEVER store scm credentials and ALWAYS use the cached credentials we provide, we would like to set this option once, globally, rather than have it set on every project.