Remove ActionMailer as a dependency

[coorasse]

Closes #76

This PR aims to remove ActionMailer as a dependency so solid_errors can be used also in Rails Apps that don't have it as a dependency. I find solid_errors a perfect solution for my smaller projects, and many of them don't actually send emails so require "action_mailer/railtie" is commented out in application.rb

[coorasse]

I added it as a development dependency anyway

[coorasse]

Zeitwerk of course complains if the class is not defined. I looked if I could exclude only this class from eager_loading and requiring it manually only if ActionMailer::Base is defined, but I did not figure out how.
This is the approach I figured: I always define it, and raise if ActionMailer is actually not there. I don't really have performance consent, and we should never reach this point anyway (see below).

Yes, it smells a bit to be honest, so I could investigate more if you think I should.

[coorasse]

This is what I'd do: immediatly check if we want to send emails but ActionMailer is not there: show a nice error message and explain how it can be fixed.