Override getIdealConditionsLimit and make it configurable

Updated patch version based on #10, but with some changes. No interdiff as patch from #10 wasn't applying to 2.x branch.

Changes:
added upgrade path - it sets the value to the current 100 items to preserve behaviour, default value for new installations will be 600 as I figured this number better represents "ideal conditions" and safe for both URL and tag type purges
changed description of the limit on settings form to be more explicit about the meaning

The last submitted patch, 14: cloudflare-3132912-14.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

Changes:

• fixed test
• rename ideal_conditions_limit => purge_limit to be more meaningful
• moved purger settings into separate section on the settings form
• improved help text of the new setting

Have been using this patch for a long time in production for multiple sites now without issue. Looks good and works a treat

While my review above that this works well stands, I've noticed 🐛 Excessive Tag Hash Collisions RTBC introduced a new config schema and form for the cloudflarepurger sub module where as this is using the cloudflare settings.

I think the config here actually logically lives in the new sub module schema? Its a bit messy as the purger would then need to reply on both the cloudflare and cloudflarepurger config.