Update changelog format

[smichel17]

Why:

• https://keepachangelog.com has a good, standard, format for changelogs that I like better than the current format.
• The current way depends on an external library
  • I'd like to limit the number of unnecessary dependencies
  • The library has no commits since 2015. It might not need any, but I'd prefer not to risk having unmaintained dependencies if something does come along that needs updating.
• The current way needs some work to style it correct when using the dark theme, anyway (#232).

Steps:

• [x] Convert changelog to a simple markdown file (no content changes)
• [x] Rewrite changelog to keepachangelog.com format
• [x] Replace the current changelog library with a simple dialog containing the raw markdown
  • There's currently no general purpose Android library for rendering markdown, but raw markdown is readable enough.