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.
* Added custom Message-ID and In-Reply-To headers for conversation reply emails
* Added new global config for the default domain (This is used in the above headers)
* Added migration to run the config loader to load the new global config value
* The subject of the conversation reply mailer was made static (This is required for threaded emails)
* Added required tests
* Renamed concern from Feature to Featurable
* Feature: Installation config (#839)
* Added new model installtion config with corresponding migrations and specs
* Created an installation config yml (key value store model)
* Created a config loader module to load the installaltion configs
* Added this to the config loader seeder
* Changed the account before create hook for default feature enabling to use the feature values from installtion config
* Renamed the feature concern to Featurable to follow the naming pattern for concerns
* Added comments and specs for modules and places that deemed necessary
* Refactored config loader to reduce cognitive complexity (#839)
