[api.keyman.com] Update deprecation flags in database to make querying faster and simpler

[mcdurdin]

Currently, we mark deprecation with the deprecates relation. This gives us one direction. However, this requires a secondary query to determine whether a keyboard is deprecated, which is not ideal for searches. We should build the corresponding relation for each keyboard as a deprecatedBy relation.

Then we need to update the search and cloud APIs to flag deprecated keyboards.

This work comes out of keymanapp/keyboards#374.