Generator: Mailer and styles

[dixpac]

Testing only mailer wasn't working from the default rails generators, so
this PR adds mailer also.

Also, fixes pointer on the submit button and small fix on controller h1
style to match the one from the scaffold index.

[dixpac]

@dhh I know we talked about splitting, because I'm a bit stubborn :) and until possible split only generator that raised error was mailer so this fixes it.

I've tried all rails default generators and they seem to work fine:

Tried also with rspec just in case, works fine also.

View component gem is another issue, as it stands we will need to contribute to view_component to support tailwindcss template_engine.