Rolling releases in v11

This is a big planned changes, comments are welcome!

0003843: Review documentation talking about feature planning and release cycles - Composr CMS feature tracker

This is an interesting change! I was just thinking about this line in particular earlier this weekend:

"The negative would be the need to stay on top of compatibility changes just to keep Composr patched, if you happen to have a customised theme or overridden code…"

Personally, I've found that I'm at that stage already, even with minor patch releases. 10.0.26 broke my forums because I had customized cns_forumview.php for my simplified URL scheme.

Something that would be incredibly helpful to me is a built-in diff tool as part of the built-in updater. If Composr could detect when one of the updated files is being overridden in sources_custom, show me my current custom version next to the new version, and let me choose which updated lines to bring in (or which custom lines to keep), it would make me feel a lot safer about doing upgrades. It might still take a few minutes, but at least I'll get to proactively review the changes, rather than having to go and click around the website after updating to make sure nothing important is broken.

I could, and absolutely should, be doing a "diff" of the updates manually, but of course that would take a while and I don't always do that. If things are going to move to a rolling-release model where the routine updates get bigger, this would become even more useful.

Automatic diffing is very tricky, but definitely something I want to give a lot more thought.