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.

spec/mailers/conversation_reply_mailer_spec.rb

@@ -97,7 +97,7 @@ RSpec.describe ConversationReplyMailer, type: :mailer do
         account = conversation.account
         account.domain = 'example.com'
         account.support_email = 'support@example.com'
-        account.domain_emails_enabled = true
+        account.enable_features('inbound_emails')
         account.save!
       end