Make mail subject and content customizable

[takezoe]

It's nice if mail subject and content is customizable in the admin console.

[exedx7]

agree, minimal effort would be adding customizable prefix,
currently prefix is "[group|user/project]", the prefix can be "PFX[...]", where PFX is a global property
defined in gitbucket.conf file and to be maintained in future gitbucket release on "system administration->system settings" page.