Fix for `smudge-api-oauth2-callback' not updating.

[cole-brown]

Bug: smudge-api-oauth2-callback' is a defconst, so if the defcustoms it uses to create its value are not up-to-date /before/ "smudge-api.el" is loaded, smudge-api-oauth2-callback' will be an incorrect value.

Fixes:

• Added `smudge-api-oauth2-callback-url' defcustom to complete the set and make callback fully customizable.
• Removed smudge-api-oauth2-callback' defconst and replaced it with the function smudge-api-oauth2-callback-url'. This will just build the full callback URL from the defcustoms.
• Updated/added docstrings for what is going on and/or what should happen with all the `smudge-api-oauth2-...' variables.