Add constants

[mohagan9]

Describe the feature request
Should be in the settings modal for easy access.

Constants are useful because there can be magic strings scattered around the builder otherwise, such as a base URL.

1. A warning message should advise against setting credential keys and passwords.
2. A constant cannot be deleted if it is in use, and links should be provided to the screen that uses it (like data queries)
3. The existing internal constants like the built-in roles, should be listed as readonly in this constants section