* Implemented sendmail. This piggybacks on existing configuration to keep the change simple

* Changed privicy of new sendSMTP and sendSendmail functions

* Fixed Lint errors

* Seperated SMTP and sendmail into their own senders

* Making new structs private as they should not be used externally now

* Added sendmail setting to ini file

* Minor code cleanup

Signed-off-by: Michael de Wit <mjwwit@gmail.com>

Integrated optional bindata for the templates

* Lint models/release.go

* Lint models/ issue_label, issue_mail & mail.go

- Update import paths from github.com/go-gitea to code.gitea.io
- Fix import path for travis

See https://docs.travis-ci.com/user/languages/go#Go-Import-Path

It is the FROM field in mailer configuration that needs be used,
not the USER field, which is for authentication.

Closes https://github.com/gogits/gogs/issues/3615

- Fill Milestone and Assignee field when available in webhook payload