Redesign details meta box

This new one looks much similar to WP post meta box
Moved E-Mail account dropdown to top
Declaring variables in local scope

Currently variables are declared in global scope rather than local scope.
They are not required globally.
Merge pull request #122 from VarunAgw/meta-box-details

Redesign Details metabox to be more visually consistent with Core.
Using a class rather than ID to get save button
Showing permissions filter using tables
Renamed SupportFlow_Email_Accounts class reference
Create get_email_accounts() to get E-Mail accounts array
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
Check if atleast one E-Mail account exists before showing thread creation page
Using post owner label as 'Unassigned'  when there is no owner
Merge pull request #123 from VarunAgw/refactor

Refactor accounts, permissions, notifications data into accessors.
Using a class rather than ID to get save button
Some E-Mail accounts were not displayed publicly due to a bug

Earlier the code was not implemented correctly causing several E-Mail
accounts to show up
Merge pull request #126 from VarunAgw/bugs

Correctly check if a user has permission to access an account.