Enable mail template customization

[kolaente]

It currently is not possible to customize email templates by using the customize system. I looked in the code, it seems like Gitea does not look for customized templates.

Relevant bit (among others): models/mail.go:167

[FalconWu2017]

The current mail template is too low, and I would like to customize it too.

[zeripath]

I think it actually does look for customisable templates.

[lunny]

I think @zeripath is right. I haven't test it, but see https://github.com/go-gitea/gitea/blob/master/modules/templates/static.go#L174 . Customize mail template should be OK.

[small75]

the mechanism of customization works