Improve user-friendliness of mail settings

[franzliedke]

Continues flarum/framework#1169

Add per-driver help texts (e.g. with example values), both a generic text and for each of the various input fields.

This can be done fully in the frontend, by interpolating the mail driver name to generate I18n keys. Care needs to be taken to do this conditionally as there might not be any text necessary for some of the fields.