Show a dash sign if E-Mail account associated with ticket is deleted in all thread page
Corrected variable spelling

$email_accoun_id => $email_account_id
Load E-Mail account data only in new thread page

In "Details" meta box
E-Mail account dropdown is only shown in new thread page so these settings
should be load only in that case
Created SupportFlow_Email_Accounts->get_email_account($id) method

Return a single E-Mail account
Test if E-Mail account already exists for thread

This is a server side check. It prevent updating of post E-Mail account if it is already set
Prevent adding new replies to thread with associated E-Mail account deleted

Current approach will disable reply box, pre-defined replies combo, attach
file button,..It will still allow to change other settings There is also a
server side check for it when saving posts
Merge pull request #146 from VarunAgw/accounts

Disable threads when their associated e-mail account has been deleted.

Fixes #139