Validate account info before storing in the local system

[le-git-imate]

Description of issue or feature request
The account info (GitHub/GitLab credentials) can be validated at the very beginning along with the key setting. Currently, le-git-imate stores account info without any check. This issue could be addressed in accordance with #1.

Current behavior

• Account info (i.e., username, password, token) is not validated at the setup step.
• The extension checks the account info as part the commit creation.

Expected behavior

• Validate GitHub/GitLab account info. It should be done for username, password and token.
• If the account is not set, the initial window should display a proper message asking user to import the credentials.
• That's should be fixed for both GitHub and GitLab.

Alternative solution

• Remove the entire initial check and do it later when the server and the commit type are detected.