Remove avatar location filter after handling avatar uploads

BP filters upload_dir when handling avatar uploads, to ensure that avatar files
are stored in the proper directories. This changeset ensures that the filter is
removed after use, to ensure the integrity of calls to wp_upload_dir() later on
the crop page (after the avatar upload has been handled).

Fixes #4574
POT, generated from r6812
Introduces bp_custom_profile_edit_fields_pre_visibility hook

This new hook allows for plugins/themes to add edit fields that appear before
the Visibility section on the edit/registration screens.

Fixes #4847

Props dontdream
Whitespace
Improve syntax in members/single/messages/messages-loop.php

Including the close quotes in the scope of a conditional resulted in valid
markup, but created problems with some syntax highlighting software.

Fixes #4842

Props MacPresss
POT, generated from r6816
When favoriting an activity item, only attempt to add My Favorites tab if Favorites tab is not already present

This prevents adding My Favorites in the context of a member/group activity
stream, while allowing the tab to be added on the global Activity directory
as intended.

Fixes #4849
Corrects selector from previous commit. See #4849
Use #buddypress instead of #content when appending more activity items via AJAX

See #4845
Use #buddypress instead of #content when detecting AJAX pagination clicks

Fixes #4845
Don't load bp-legacy theme compat if current_theme_supports( 'buddypress' )

Fixes #4846
POT, generated from r6822