Update the Rails mailing list URLs to new discuss discourse URL

Fix ActiveStorage::Blob → ActionText::TrixAttachment conversion

Adds an attachment button to the Trix toolbar that improves overall file upload usability, especially on mobile devices where files can't be dragged / dropped.

References:
- https://github.com/basecamp/trix/releases/tag/1.2.0
- https://github.com/basecamp/trix/pull/619
- https://github.com/basecamp/trix/issues/582

Ignore SQLite3 database files generated by parallel testing

- The sanitizer has been changed to safe_list_sanitizer.
- deprecate white_list_sanitizer

Fixes that file attachments without captions would not be represented in plain text generated from rich-text content, causing ActionText::RichText#present? to return false.

Closes #36607.

Fix that an ActiveRecord::RecordNotUnique error would be raised when saving rich-text content with the same file attached multiple times.

- Allow configuring the sanitizer and its options
- Split attachment rendering and sanitizing helpers so each can be overridden by applications

Assigning a has_one association for a persisted record saves the change immediately, so attempting to read a rich-text attribute on a persisted record without a corresponding ActionText::RichText would eagerly create one. Avoid assigning the rich text association to fix.

Closes #35159.

* Update RAILS_VERSION
* Bundle
* rake update_versions
* rake changelog:header

If the [`action_text.helper` initializer][0] runs after
`ActionController::Base` has been loaded, but before the
`rails-html-sanitizer` gem has been `require`d, then the reference to
the constant `Rails::Html` in the body of the
`ActionText::ContentHelper` module raises an `uninitialized constant`
exception.

[0]: https://github.com/rails/rails/blob/21703382393c87212c27c988420ee5c133c1aa9f/actiontext/lib/action_text/engine.rb#L31-L35

Without this change, `store_translations` silently fails when available
locales already initialized.

Ref:
https://travis-ci.org/rails/rails/jobs/497615616#L6846
https://travis-ci.org/rails/rails/jobs/497605027#L6856

I implemented Foreign key create in `create_table` for SQLite3 at
#24743. This follows #24743 to implement `add_foreign_key` and
`remove_foreign_key`.
Unfortunately SQLite3 has one limitation that
`PRAGMA foreign_key_list(table-name)` doesn't have constraint name.
So we couldn't implement find/remove foreign key by name for now.

Fixes #35207.
Closes #31343.

In MySQL, the text column size is 65,535 bytes by default (1 GiB in
PostgreSQL). It is sometimes too short when people want to use a text
column, so they sometimes change the text size to mediumtext (16 MiB) or
longtext (4 GiB) by giving the `limit` option.

Unlike MySQL, PostgreSQL doesn't allow the `limit` option for a text
column (raises ERROR: type modifier is not allowed for type "text").
So `limit: 4294967295` (longtext) couldn't be used in Action Text.

I've allowed changing text and blob size without giving the `limit`
option, it prevents that migration failure on PostgreSQL.

This reverts commit 30f666f8.

Follow up #34956.