[imorland]

Fixes: no issue filed. Follow-up to #5321.

Changes proposed in this pull request:

The OpenSearch description always sent searches to /?q={searchTerms}. That only works when the discussion list is the homepage. With a different homepage, such as Tags, the search opened that page and ignored the query. When the discussion list is the homepage, the frontend has nothing at /all, so that address can't be used unconditionally either. Searches now go to / when the default route is /all, and to /all otherwise. A tag page set as the homepage therefore gets a forum-wide search rather than one inside that tag.

Also:

• The description is served as application/xml. Browsers download application/opensearchdescription+xml when the address is opened rather than show it. Neither Firefox nor Chrome checks the type when it loads the description, and Discourse serves the same type. The skeleton's .htaccess also expires application/xml immediately, where an unlisted type got a month.
• <Tags> and an example <Query> are added, as the OpenSearch 1.1 spec suggests. The example uses the forum's title, the term most likely to have results in any language.

Reviewers should focus on:

• Checked on a forum with Tags as the homepage, in fresh profiles. Firefox 158 added the engine with …/all?q={searchTerms}, and a search from its address bar showed results. Chrome stored the same address.
• moz:SearchForm stays at the forum's root. Firefox uses it as the engine's main page, opened with no terms; it isn't a search template.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (No frontend changes.)
• Frontend changes: tests are green (run yarn test in js/). (No frontend changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No frontend changes.)
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (No database changes.)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.