refactor: better engine mounting

[adrianthedev]

Description

This PR refactors the way the Avo engine is mounted. Same for other Avo plugins.
This should significantly improve the way we recommend users to mount Avo when they are using the scope option.
I guess we could even remove the self.mount_avo_engines configuration option at some point.

# Other engines use this API to self-mount themselves
Avo.plugin_manager.mount_engine Avo::Collaborate::Engine, at: "/collaborate"

# like so...
module Avo
  module Collaborate
    class Engine < ::Rails::Engine
      isolate_namespace Avo::Collaborate

      initializer "avo-collaborate.init" do
        ActiveSupport.on_load(:avo_boot) do
          Avo.plugin_manager.register :avo_collaborate

          Avo.plugin_manager.mount_engine Avo::Collaborate::Engine, at: "/collaborate"
        end
      end
    end
  end
end

TODO

• make mount_avo take a at: argument
• cleanup
• tests
• update all other plugins to self-mount themselves
• docs

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. Step 1
2. Step 2

Manual reviewer: please leave a comment with output from the test if that's the case.

[codeclimate]

Code Climate has analyzed commit a3cf5ce and detected 0 issues on this pull request.

View more on Code Climate.

[adrianthedev]

This should still be here with a warning that it's not working anymore.
Deprecation warning.

Maybe

[Paul-Bob]

Upgrade guide:

• Remove all references to mount_avo_engines
• Remove all references to Avo.mount_engines
• Replace mount Avo::Engine, at: Avo.configuration.root_path with mount_avo

[adrianthedev]

don't we still need this for reloading the code?

[adrianthedev]

Let's run this only in development

[adrianthedev]

We need comments with examples.

[adrianthedev]

And maybe use scope at do

[adrianthedev]

Let's run this only in development

[adrianthedev]

We don't need this in production.
Development will always run it twice.

[adrianthedev]

Let's add a setter method to raise an error with docs link.

[adrianthedev]

Same. Raise and error with docs links.