Migrate changesets history RSS feeds

[Zaczero]

The goal is to replicate the behavior of:

• https://www.openstreetmap.org/history/feed
• https://www.openstreetmap.org/user/NorthCrab/history/feed

Additional requirements:

• The code should reuse parts of existing HTML responses (without the /feed suffix): querying, filtering, etc.
• The data formatting should be contained in app/format/api06_changeset_rss.py file
• Take inspiration from api06_note_rss.py

[tomekrzymyszkiewicz]

I've read how the note RSS feed is created, and I will try to implement that for changesets.