Create "adopt-me" taxonomy

[samuelsidler]

Currently, we ask plugin authors to add the "adopt-me" tag to their plugin if they are no longer able to work on their plugin. Since we're removing free-form tags from the plugin directory, we need to bring this over as a separate "taxonomy" though it probably technically doesn't need to be a taxonomy.

For plugin authors, this will show as a check box in the plugin admin with the text:

I am no longer developing this plugin. Please mark it as available to “adopt” by another developer.

On the front-end, we'll work on a solution in #1763.

I think we need to complete this ticket and migrate all current adopt-me plugins before going live with the plugin directory. However, the front-end can be created later.

[dd32]

I don't think we need a separate taxonomy for this, I think a Adopt Me term in the plugin_section taxonomy could be used instead.
Plugin Admins would be able to flag a plugin as adoptable through the standard UI's, and we could add a form which allows an author to flag it as such.

[samuelsidler]

Yeah, I didn't think it needed to be a separate one, but wasn't sure either way. :)

[obenland]

In 3357:

Plugin Directory: Migrate plugins that are up for adoption to new taxonomy.

See #1762.

[dd32]

remaining front-end work can be done in #1763