Chore: Feature lock email settings in UI (#1065)

* Chore: Feature lock email settings in UI

The email settings under account settings needed to be
feature locked in a way different from teh current way for it
to be enabled for accounts in a self hosted scenario.

Some refactorings were also done along with this change.

1. There was a feature flag defined in code in account model called
domain_emails_enabled was used to check if the inbound emails was
enabled for the account. But there was already a feature flag called
"inbound_emails" defined in features.yml. So changed to use this to
check if inbound emails are enabled for an account.
2. Renamed and re-purposed existing `domain_emails_enabled` to
`custom_email_domain_enabled` to use for feature toggling the UI
for email settings.
3. To enable & disable multiple features using the featurable concern
we were passing an array of values. Changed this to accept a comma
separated set of values.

* Chore: Feature lock email settings in UI

Fixed the specs for accounts controller & removed
unneccessary code from Account seetings component in UI

* Chore: Convert newlines to <br>s

Removed the layout used while sending replies in
conversation continuity.

Converted the newlines in the messages to <br/> tags
for the correct HTML rendering.

* Chore: Bug fix in reply email domain

Renamed the function custom_email_domain_enabled  to
inbound_email_enabled.

Fixed bug on setting reply emails's domain.

app/javascript/dashboard/i18n/locale/en/generalSettings.json

@@ -24,8 +24,8 @@
         "ERROR": ""
       },
       "DOMAIN": {
-        "LABEL": "Domain",
-        "PLACEHOLDER": "Your website domain",
+        "LABEL": "Incoming Email Domain",
+        "PLACEHOLDER": "The domain where you will receive the emails",
         "ERROR": ""
       },
       "SUPPORT_EMAIL": {
@@ -33,14 +33,9 @@
         "PLACEHOLDER": "Your company's support email",
         "ERROR": ""
       },
-      "ENABLE_DOMAIN_EMAIL": {
-        "LABEL": "Enable domain email",
-        "PLACEHOLDER": "Enable the custom domain email",
-        "ERROR": "",
-        "OPTIONS": {
-          "ENABLED": "Enabled",
-          "DISABLED": "Disabled"
-        }
+      "FEATURES": {
+        "INBOUND_EMAIL_ENABLED": "Conversation continuity with emails is enabled for your account.",
+        "CUSTOM_EMAIL_DOMAIN_ENABLED": "You can receive emails in your custom domain now."
       }
     }
   }