Chore: Move some email configs to ENV variables (#1064)

For the outgoing emails which has dependency on the incoming
part as well like the conversation continuity part, some of the
config variables used were entirely based on the account attributes.

But this is not true in case of self hosted situations where you have
multiple accounts and have a common config for incoming emails.

So moved out some of the attributes entirely dependednt on the account
to ENV with a fallback to the Global config.

Also, with this changes the name of the agent will be shown in the
email clinet with in the conversation rather than just the support
email address. This has a huge UX impact on the cutomer.

Modified all the necessary unit tests to reflect these changes.

Updated the .env.example file for the new ENV variable.

.env.example

@@ -40,7 +40,8 @@ SMTP_AUTHENTICATION=
 SMTP_ENABLE_STARTTLS_AUTO=
 
 # Mail Incoming
-
+# This is the domain set for the reply emails when conversation continuity is enabled
+MAILER_INBOUND_EMAIL_DOMAIN=
 # Set this to appropriate ingress channel with regards to incoming emails
 # Possible values are :
 # :relay for Exim, Postfix, Qmail