Added mailer configs

[bernd-wechner]

Having configured the mailer, I figure the mailer configs should be in run.sh as well, to make them easily discoverable.
Added a few comments as well and export each variable as it's defined rather than in a separate statement.

[rokdd]

I thought that makes no sense, to export the vars when they are not properly set up. So my intention was to make a difference between:

• default: vars with default values and works also without editing
• optional: vars which need to be individual set because there is no default

[bernd-wechner]

I guess we could comment them out by default. Really just added them heard rather that .joplinrc. Can push a version with commented out lines thoigh one might argue all of the settings should be commented out until set properly. Or maybe the defaults can simply be blank rather than examples, or if the default for MAILER_ENABLED is 0 that might cover everything set it to 1 only once the others have sensible values. Hmm not sure. Not fussed really, was just sharing where I did set them I guess.

Technically, given all run.sh does is set these it could maybe diappear, and the service just runs a config file source then npm --prefix packages/server start. And sample config file is provided. Perhaps the ocnfig file shoudl in fact be /etc/config/joplin Random thoughts. The PR was as much intended to provoke discussion asa anything I guess.