Make sure the way the single group canonical stack is set is right

[imath]

Working on making bbPress ready for the BP Rewrites API, I've noticed a misuse of the bp_groups_get_path_chunks() function when setting the action_variables key of the group's canonical stack.

As the URL chunks were missing the current action, one of the action variables was wrongly removed, the single view of a topic was wrongly redirected to an URL missing the topic URL chunk before the subject of the topic.

A PR will follow to fix this annoying issue.

If the manage context does not need the current action to get the customized slugs for action variables, the read context does.

Moreover, as the current group is not set during the creation process, we don't need to get customized action variables for the create context.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8940

[imath]

In 13521:

Ensure the way the single group canonical stack is set is consistent

If the manage context does not need the current action to get the customized slugs for action variables, the read context does.

Moreover, as the current group is not set during the creation process, we don't need to get customized action variables for the create context.

Fixes #8940
See #4954
Closes ​https://github.com/buddypress/buddypress/pull/134