[4.0] Make sure routes can be generated even if developer uses phpcbf; don't generate route and sidebar item if it already exists

[tabacitu]

Fixes #2536

Makes the route generator check for these line contents, in this order, and places the route BEFORE that line:

• // end of generated Backpack routes; DO NOT delete or modify this comment;
• }); // this should be the absolute last line of this file
• });
• nothing?! then the last line of the file - 1;
• no lines?! ok, then return 0;

In addition:

• it checks that the route doesn't exist before writing content;
• sidebar also checks that it doesn't exist before writing content;

Now if you're trying to generate a CRUD that already exists, the end result is similar with the model, view, controller:

[scrutinizer-notifier]

A new inspection was created.