Fix RT 84403 - 'Security problem: missing "start" mode dumps ENV to output page'

This pull request resolves the issue raised in https://rt.cpan.org/Public/Bug/Display.html?id=84403

Application.pm

A new runmode named ```no_runmodes``` is now called  rather than ```dump_html``` when no run modes are specified. This returns a message to the user reporting the problem, without expositing anything which may raise security concerns. The POD has been updated, asking the user to think about potential security issues when calling dump_html.

basic.t/TestApp.pm

Add tests for the new runmode 

load_tmpl_hook.t

Test required an update as it was dependant on Application.pm returning the output of ```dump_html```

In addition to the changes above, some very minor changes to the indentation.

If there are any issues please let me know.