Using SupportFlow_Email_Accounts::get_email_accounts() to get array of E-Mail accounts

Currently it is accessing them directly from database
Creted SupportFlow_Permissions::get_user_permisssions() to get array of user allowed tag/E-Mail account
Using SupportFlow_Permissions::get_user_permissions_data() to get permissions for a user

Earlier code was directly querying it from database
Created SupportFlow_Email_Notifications::get_email_notifications() to get tag/E-Mail account user opted to receive E-Mail notifications
Using get_email_notifications() to get E-Mail notifications rather querying database directly
Add option to retrieve only existing E-Mail accounts
Show only existing E-Mail accounts in New thread combo box