Fix e-mail template overrides by adding the "custom" prefix server-side

[garyttierney]

Separate template replacement from adding the initial "custom" prefix,
so that server-side templates (namely e-mails) can also have overrides.

[garyttierney]

Bit of context to this: the custom_ prefix is only ever registered on the client. Since telescope-email manages templace replacements itself it never finds the prefix.

To reproduce, create and add a new "custom_emailWrapper.handlebars" to a package, any changes made won't be reflected on your server.

The solution is to move Telescope.config.addCustomPrefix("custom_"); to a JS file required on both the client and server.

[SachaG]

Makes sense, thanks!