Search tools on metadata

For now, metadata are only used for fulltext/keyword search. There is a lot more we can do. This meta-task will track ideas and progress of their design.

Ideas:

• Better full-text search (for now, it's keyword-based)
• Search per field
  • Single field
  • Combining fields
• Filter results to a subset of metadata file types ("mappings")
• Stats on frequency of values for each field (e.g., license, ...)
  • on all metadata
  • on a subset of metadata (filtered by a search query)
• Metadata-only search (right now it's always in OR with URL-based search)

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Migrated from T1523 (view on Phabricator)