Add filters to edit the URL used to redirect an activated account

[imath]

I'm wondering why we are using two different ways to get the activation URL when an account is activated?? Is there a specific reason to use bp_get_root_domain() . '/' . $bp->pages->activate->slug instead of bp_get_activation_page() when there's an activation error?

The patch suggests to use the same template tag to get the activation URL and add filters to let advanced users eventually edit this redirection.

[espellcaste]

I don't know why the two different URLs, but patch looks good.

[imath]

In 13125:

Introduce filters to allow custom redirection URL on activated account

• bp_members_invitations_activation_errored_redirect can be used to customize the URL to redirect the user to if activating the invitation failed.
• bp_members_invitations_activation_successed_redirect can be used for the same purpose but on a successful activated account.
• bp_members_action_activate_errored_redirect can be used to customize the URL to redirect the user to if activating the account failed.
• bp_members_action_activate_successed_redirect can be used for the same purpose but on a successful activated account.

NB: this commit also makes sure the bp_get_activation_page() is used to build the activation page url instead of rebuilding this URL from the activate actions.

Fixes #8542