Don't load BP_Group_Extension subclass if parent does not exist

Fixes #420

Props @dcavins
Readme
Correct the logic of the is_protected cache check

We cache the remote GET request used to check whether attachments are protected,
with this cached value being checked before moving on to do the GET request.
The logic that is used to verify whether a cached value has been stored was
backward, however, resulting in some very slow lookups.

Many thanks to @brooks1man for help debugging this issue.

Fixes #422
Readme
Better parsing of submitted bp-docs settings in groups

bp-docs settings include group-enable, which is represented in the markup by
a checkbox. This means that when the checkbox is unchecked, no value for group-
enable shows up in the POST array. As a result, when Docs is disabled for a
group, no value is saved in the settings array. Since the introduction of
bp_docs_get_group_settings() (and its internal default parsing), this has meant
that groups without a value for group-enable incorrectly get Docs turned on.

We fix this by ensuring that all bp_docs_get_group_settings() keys are filled
in saved settings arrays.

Fixes #424
Readme
Bump version numbers to 1.8.2
Merge branch '1.8.x' into folders