Improve user-friendliness of mail settings

**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.

Start by locating the frontend mail-settings form and reviewing its existing internationalization keys. Check how the mail driver name and input fields are represented, then verify that generic and driver-specific help text appears only where translations exist. Done means each relevant mail setting has appropriate example or explanatory text without empty help areas.

Written by the indexing model from the issue text.