Adds _x() context for sprintf() text in bp_docs_get_item_term_id()

Fixes #322
When filtering doc permalinks, rebuild URL to cut off all parent slugs

The previous code for generating canonical Doc URLs attempted to slice off
the slug of $post->post_parent when it was found. But this did not work for
Docs that were nested more than one level deep.

The new code enforces a totally flat URL structure by rebuilding the URL based
on the Docs archive link.

Fixes #308
Fix tab indexes for Doc Create mode. See #311
Readme update
list-style-type: none for directory attachment drawer

Fixes #323

Props @schwarzaufweiss
Allows the attachments component to be disabled
Readme
Don't show Delete Attachment button in Read mode.

Fixes #321
Use proper API function in test_bp_docs_is_existing_doc()
Updates de_DE
Bumps version to 1.4.2
Merge branch '1.4.x'

Conflicts:
	loader.php
	readme.txt