Improved release notes.

- Format markdown with headers and bullets.
- Split out comments with more than one FIX or FEATURE
- Escape a few characters that disrupt formatting
FIX: Make shortcut 'c' global for creating a topic

https://meta.discourse.org/t/keyboard-shortcut-c-create-a-new-topic-does-not-work-globally/18975
FIX: Drupal QA was missing some comments
Delete MIGRATION.md
FIX: Moderators/Admins cannot delete other user's bookmarks
Call open directly to avoid deprecation warning
FIX: Do not show unconfirmed users in search results
Merge pull request #2916 from cpradio/pr-unconfirmed-in-search

FIX: Do not show unconfirmed users in search results
Merge pull request #2909 from mcwumbly/release-notes-improvements

Improved release notes readability / formatting
UX: Improve username readability in search results

Avoid lower-case usernames in search results output to improve readability and accuracy - avoid users reading words that don't exist.
UX: Improve username readability in search results

Avoid lower-case usernames in search results output to improve readability and accuracy - avoid users reading words that don't exist.
FIX: email replies to closed topic should not be accepted
Merge pull request #2919 from techAPJ/patch-2

FIX: email replies to closed topic should not be accepted
Merge pull request #2917 from DeanMarkTaylor/patch-1

UX: Improve username readability in search results
Update list.rss.erb (unescape tag)
FIX: search was rendering incorrectly in safari mac
apply same fix to mobile
FEATURE: add max_reply_history to limit number of replies
that can be expanded, when clicking "in-reply-to"
Merge pull request #2920 from yomybaby/patch-1

Update list.rss.erb (unescape tag)
FIX: email replies should not be accepted for deleted topics