Allow custom login and error template

[growse]

The html template is currently hard-coded in templates.go. It'd be useful if there was an application switch that allowed specifying a custom template file for both the login and error pages.

[jehiah]

Great idea. The default templates could use some work too.

[mikz]

I would like to have the login in a popup, because we have the page in iframe and google sends x-frame-options: deny so it does not work.
Passing custom template would be awesome.

[timurb]

I have not tried that but I think you can specify a template for an error page by nginx error_page directive: http://nginx.org/en/docs/http/ngx_http_core_module.html#error_page
Probably this would also work for a sign in page.

[jehiah]

@timurb @mikz @growse I've updated the default templates quite a bit and want to know if the changes in #36 resolve this for you.

There is some ongoing conversation in #15 about this issue, so i'm going to close this and encourage you to chime in there if this is still of value with you.