Support loading multiple recordings and/or blueprints in web-viewer

[emilk]

What

• Closes Support for working with blueprint files #5294

This adds support for passing multiple .rrd and/or .rbl files to the web-viewer by repeating the url query parameter.

TODO

• Test this

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

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