Activate and Register pages should be auto-created when registration is opened

[boonebgorges]

If you activate BP without having WP registration turned on (Settings > "Anyone can register" in single WP, Network Admin > Settings > Registration on multisite), the BP pages for Activate and Register are not auto-created. If you open registration at a later date, the pages still are not created - you're only shown a notice that you'll have to create them manually.

The proposal here is that Activate and Register should be created automatically when registration is enabled on an existing BP installation.

Note that this ticket will become moot if we adopt rewrite rules and do away with bp-pages in a future release.

[mercime]

See ​https://buddypress.trac.wordpress.org/ticket/7193#comment:3

[mercime]

Related: #3599

[r-a-y]

Closing this in favor for #7193.