Moved explanation of `--mountable` option up

The example code uses `--mountable`. In previous version this was followed by an explanation of the `--full` option, which doesn't make sense. Later on in the text this is better explained. I moved this explanation up without repeating too much.
Prevent state leak.
Remove redundant code.
[ci skip] correct output for asset_helper methods
    see cc255d3
Added missing colon in asset manifest JSON.
Add to guides app and helper objects on command line

[ci skip]
Add rails:update to the upgrading guides

[ci skip]
Add warning for `rescue_from Exception` and `rescue_from StandardError`

[ci skip]
Merge pull request #15537 from tgxworld/fix_state_leak

Fix state leak.
Merge pull request #15809 from maurogeorge/guides-console-app

Add to guides app and helper objects on command line
docs, configurations method is listed in the docs. [ci skip]

The :singleton-method: directive is printed in the docs.
This directive is not necessary as the method is documentable just fine.
Make dependencies.rb add a name to NameError
Add regression test for NameError#name
Replace x.sort_by!.select! with x.select!.sort_by!

The latter has the same speed as the former in the worst case
and faster in general, because it is always better to sort less items.

Unfortunately, `routes.select!{...}.sort_by!{...}` is not possible here
because `select!` returns `nil`, so select! and sort! must be done
in two steps.
Merge pull request #15764 from arthurnn/dependencies_name_error

Make dependencies.rb add a name to NameError
Return a null object from `AttributeSet#[]`
[ci skip] Add Tip for change_column_null and change_column_default in guides
Merge pull request #15624 from aditya-kapoor/add-warnings-on-guides

[ci skip] Add Tip for change_column_null and change_column_default in guides
:nail_care: wrap the tip from #15624 at 80 chars [ci skip]
Make the isolated tests run on JRuby

As there is no forking on JRuby, we need to spawn sub-processes to make
the tests run in isolation.

Previously, we were defining globally env variables and running the test
file through backticks and delete these variables once the test ran.

Now, we simply rely on IO.popen as this is cross-platform and the env
variables are available during the child-process execution only so there
are no race conditions.

[Ben Browning & Robin Dupret]