Improve user registration workflow for administrators

[r-a-y]

Since 2.7 is all about UX, we should improve the user registration workflow for administrators.

When registration is disabled and the site admin opens up member registration, if the registration and activation pages are not created, we should show an admin message saying that these pages are not created yet.

To be fair, we do add this message:

The following active BuddyPress Components do not have associated WordPress Pages: Activate, Register.

But, that is kind of vague.

We already add an admin message if permalinks are not set up, so adding another admin message here seems okay to me.

Currently, people assume that simply setting the registration and activation pages is enough.

This is one of the most asked questions on the support forums.

We should also remove the "Register" section on the "Pages" screen until registration is open. I brought this up here:
​https://buddypress.trac.wordpress.org/ticket/6176#comment:7

Would appreciate some feedback here before I start patching this up.

[espellcaste]

The pages created (register and activation) would be removed if one decides to disable registration later? Unlikely scenario, but just wondering.

[boonebgorges]

When registration is disabled and the site admin opens up member registration, if the registration and activation pages are not created, we should show an admin message saying that these pages are not created yet.

Better: We should go ahead and create the pages + mappings. We do this when top-level components are activated.

We should also remove the "Register" section on the "Pages" screen until registration is open.

Love it.

[mercime]

+100 @r-a-y Thank you. I was just thinking of this Pages screen this week. But more on how to add new pages in the same Pages screen for Activate and Register when registration is enabled instead of going to Pages > Add New screen :)

+100 @boonebgorges' suggestion "Better: We should go ahead and create the pages + mappings. We do this when top-level components are activated."

[mercime]

Boone's comment above related to #4826

[mercime]

Related: #3599

[r-a-y]

auto-create.patch auto creates the registration and activation pages when user registration is turned on. Includes unit tests.

• Notice how I'm using bp_core_add_page_mappings( array( 'pizza' ) )? It's because we can't pass 'register' and 'activate' since this is automatically done inside that function.)
• I've changed bp_get_signup_allowed() to simply use bp_get_option( 'users_can_register' ). I outlined the reasoning here: ​https://buddypress.trac.wordpress.org/ticket/6176#comment:7

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bp-pages.patch implements the changes to the "Settings > BuddyPress > Pages" admin page as shown in the ticket description's screenshot.

[r-a-y]

In 11154:

Members: Simplify bp_get_signup_allowed().

We only need to call bp_get_option( 'users_can_register' ) for both
single-site and multisite as multisite already filters the
'users_can_register' option:
​https://core.trac.wordpress.org/browser/tags/4.6.1/src/wp-includes/ms-default-filters.php#L20
​https://core.trac.wordpress.org/browser/tags/4.6.1/src/wp-includes/ms-functions.php#L2104

See #7193.

[r-a-y]

In 11155:

Members: Auto-create registration pages when user registration is enabled.

Props r-a-y, boonebgorges.

See #7193.

[r-a-y]

In 11156:

Admin: Do not show "Registration" pages dropdown options if registration is disabled.

Fixes #7193.