Settings form rejects valid consumer key and secret

Problem/Motivation
The settings form at admin/config/salesforce_auth/settings rejects valid Salesforce credentials:
1. The consumer secret must be all digits. Newer Salesforce connected apps issue secrets that contain letters, so they can't be saved.
2. The consumer key check is inverted. It rejects keys that contain only letters and numbers.
The error messages are also misleading ("must be minimum 5 characters").

Proposed resolution
- Accept consumer secrets made of letters and numbers.
- Accept consumer keys made of letters, numbers, dots and underscores.
- Make the error messages describe the actual rule.