DEV: Update release notes output format, and add plugin task (PR #13709)

Updates the release_note:generate task to output ### level headings, and adds a task for generating plugin release notes.

For example:

rake "release_note:plugins:generate[ HEAD@{2021-07-01} , HEAD@{now} , ../all-the-plugins/plugins/* , discourse ]"
## discourse-encrypt

### New Features

- Ability to permanently destroy the encrypted message
- Decrypt notification titles before rendering

### Bug Fixes

- Do not allow encrypted PMs to be converted
- Use Ember's debounce on stable.
- Make all lookups by username case insensitive
- Decrypt small action widget
- Email annotation that message will be deleted
- Remove order when find_each in the timer job
- Timer look when long user title
- Make sure all user identity promises are eventually resolved
- Visual improvements for the timer
- Correct encrypted_post_timer_evaluator job definition
- Do not error timer when post is already deleted
- Correct order to delete the message
- Email about encrypted message contains the topic id
- Check if arguments are valid
- Encrypt works for a user who cannot edit posts
- Eager load encrypt tables in search
- Even admin does not see/like encrypted posts
- Move lightbox decorator to after-cooked
- Lightbox images only if bigger than max_image_width/height
- Decorate decrypted images with lightbox
- Remove double data-topic-id span in quick-access-item

### UX Changes

- Improve error messages if decryption fails
- Simplify key pair reset
- Make 'Disable' red in the preferences menu
- Revise styles for exploding messages
- Replace 'reset key pair' with 'disable encryption'
- Improve enable and activate processes

### Performance

- Do not wait for debounce call if user identity is cached

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## discourse-saml

### New Features

- Add support for multiple IdP certificates.

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## discourse-login-with-amazon

(no changes)
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