Make the search query language a first class citizen

or even the only search syntax provided :

Make the textual search language of archive.s.o a first-class citizen, including:
    Simplify syntax
    Conduct UX audits and user-testing of the web search UI
Note: this does not include extending the type of data currently indexed and used for search (e.g., no filenames, no file content, etc.; they can come later/separately).

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Migrated from T3952 (view on Phabricator)