Motivation / Background

This Pull Request has been created because tags input has had some inconsistent behaviors in the past related to the string returned and the token that separated the tags in said string. Plus, it seemed a bit counter-intuitive that a tags/multiselect input returned a string instead of an array.

Closes #335
Closes #341
Closes #421

Detail

This Pull Request changes the tags input so it returns an array of strings instead of a token-separated string. It does so by relying on the default behavior of a regular rails select with multiple: true. This allowed me to simplify the input a lot, as now a couple of things, such as hidden input with blank value and tracking of the array value, are done under the hood.
The input now works almost identically as doing f.input :foos, as: :select, multiple: true, with the added bonus of being able to set value and display_name, and having the addable function defined for non-AR relation uses.
A disclaimer was added to inform of the return type and a gotcha present in multi selects that's important when using with Postgres' array column.

Additional information

With a Purchase model that has:

• A foos column of Postgres array type
  • With the following to clean up the column:

  before_save do
    self.foos = foos.reject(&:blank?)
  end

• An acts_as_taggable_on :aato_tags, from the library acts_as_taggable_on
  And an admin form with the following form:

  permit_params foos: [], aato_tag_list: []  

  form do |f|
    f.inputs do
      f.input :foos, as: :tags
      f.input :aato_tag_list, as: :tags
    end
    f.actions
  end

It looks like this:

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a concise description of what changed and why.
• Tests are added or updated if you fix a bug or add a feature.
• Documentation has been added or updated if you add a feature or modify an existing one.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature (under the "Unreleased" heading if this is not a version change).
• My changes don't introduce any linter rule violations.