Switch default collection setting to be on for all environments.

* Closes #159.
* Includes documentation updates for code changes. /cc @assaf how can we update the docs at http://vanity.labnotes.org ?

This changes the settings established in v1.4.0 and fixed in 57cd3e9a. The motivation for this is:
* To avoid confusion/support issues like #24 and https://github.com/assaf/vanity/issues/80#issuecomment-2851962
* Make it easier to setup and try Vanity in development (offering proof of functionality), before deploying it to production
* Follow Ruby gems/Rails conventions: from a brief survey, it seems like other gems do _not_ turn off by default in development/test

Some of the hassle of setting up the datastore in development is mitigated by #11. An alternative to this PR might be to separate an "enabled" setting from "collecting" where ```enabled=true, collecting=false``` allows Vanity to show alternatives/function normally, but discards data.