fix: let a link to an uploaded file open instead of routing to the index

[jamalkamaladdin]

Fixes #5012.

Changes proposed in this pull request:

Formatter marks every same-host URL as an internal link, including one under the assets path. routeInternalLinks hands that path to Mithril, and Mithril has no route for a file path and returns to the index.
A click on such a link now checks whether its href starts with assetsBaseUrl, and a same prefix leaves the click to the browser. Changed file: framework/core/js/src/forum/utils/routeInternalLinks.ts.
New test: "leaves an uploaded file under the assets path to the browser", in framework/core/js/tests/unit/forum/utils/routeInternalLinks.test.ts.

Reviewers should focus on:

assetsBaseUrl can point at a host other than the forum's own, and the existing origin check returns before the new check runs in that case.

Screenshot

This is routing behavior with no visual change.

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.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• 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).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.