Change mailing implementation

[iamareebjamal]

The current mailing implementation is too rigid and not robust.

The Link Mailing service is handling too much like templating and other things. Ideally, mail service should only care about mail and a wrapper above can be added to handle templating. Still, the rendering part should be handled by specific actors like attendance and notification email senders and not a common service

Also, there should be an option to use a mailing API like Postal to make it easy to track mails. A dynamic bean needs to be created which will return an implementation of basic mailing service based on availability of credentials and mailing API keys

• Simplify mailing implementation
• Add support for mailing APIs