add hook for media_upload_library_form()

[kevinB]

The "Media Library" tab of the Upload Media dialog includes a date filtering dropdown. The query which populates it returns only month/year combinations in which attachments were uploaded. However, that query is not filterable.

Plugins which filter backend access to attachments via standard WP_Query hooks need a way to make the month/year dropdown match those filtered results.

The corresponding patch adds query filter 'media_upload_year_month_query'

[kevinB]

query filter for month/year query in media_upload_library_form()

[scribu]

Related: #17022

[songsthatsaved]

This applies to the old media experience. Closing.