Use null column for association key types
Merge pull request #15502 from sgrif/sg-use-null-column

Use null column for association key types
Revert "Correct Documentation for asset_tag_helpers"

This reverts commit c22a253d1a72602331db1eba3e91bc945eff2346.

Reason: Our documentation assumes the assets are in the proper place and
we are using the default Rails stack.

With the default Rails stack and the assets being in the correct place
the assets helpers uses the `/assets` prefix.
have an actual `NullColumn` object and update docs accordingly.

Follow up to #15438 and #15502.

/cc @sgrif
Emit suggested generator names when not found

When someone types in a generator command it currently outputs all generators. Instead we can attempt to find a subtle mis-spelling by running all generator names through a levenshtein_distance algorithm provided by rubygems. 

So now a failure looks like this:

```ruby
$ rails generate migratioooons
Could not find generator 'migratioooons'. Maybe you meant 'migration' or 'integration_test' or 'generator'
Run `rails generate --help` for more options.
```

If the suggestions are bad we leave the user with the hint to run `rails generate --help` to see all commands.