Fix search in playback (wrong paths in alternate), see #140

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Hello,
I'm having the same issue reported in #140 and I traced it back to the way buildAlternates (in src/utils/builder.js) looks for the text data. The path of the thumbnail seems to be wrong, therefore the text is never associated with the slide/thumbnail.

The path is changed as follows (example for slide nr. 7) presentation/${presentation}/slide-7.png to presentation/${presentation}/svgs/slide7.svg

I don't know if this is an issue with my setup or a general issue, btw my configuration is as follows:
My configuration:
BigBlueButton Server 3.0.18 (3174)

ii bbb-apps-akka 1:3.0-347 all BigBlueButton Apps (Akka)
ii bbb-config 1:3.0-92 amd64 BigBlueButton configuration utilities
ii bbb-etherpad 1:3.0-9 amd64 The EtherPad Lite components for BigBlueButton
ii bbb-export-annotations 1:3.0-40 amd64 BigBlueButton Export Annotations
ii bbb-freeswitch-core 2:3.0-12 amd64 BigBlueButton build of FreeSWITCH
ii bbb-freeswitch-sounds 1:3.0-4 amd64 FreeSWITCH Sounds
ii bbb-fsesl-akka 1:3.0-116 all BigBlueButton FS-ESL (Akka)
ii bbb-graphql-actions 1:3.0-84 amd64 BigBlueButton GraphQL Actions
ii bbb-graphql-middleware 1:3.0-151 amd64 GraphQL middleware component for BigBlueButton
ii bbb-graphql-server 1:3.0-295 amd64 GraphQL server component for BigBlueButton
ii bbb-html5 1:3.0-1560 amd64 The HTML5 components for BigBlueButton
ii bbb-learning-dashboard 1:3.0-59 amd64 BigBlueButton bbb-learning-dashboard
ii bbb-libreoffice-docker 1:3.0-13 amd64 BigBlueButton setup for LibreOffice running in docker
ii bbb-mkclean 1:3.0-5 amd64 Clean and optimize Matroska and WebM files
ii bbb-pads 1:3.0-12 amd64 BigBlueButton Pads
ii bbb-playback-presentation 1:3.0-9 amd64 BigBlueButton presentation recording format
ii bbb-record-core 1:3.0-35 amd64 BigBlueButton record and playback
ii bbb-web 1:3.0-243 amd64 BigBlueButton API
ii bbb-webrtc-recorder 1:3.0-24 amd64 BigBlueButton WebRTC Recorder
ii bbb-webrtc-sfu 1:3.0-61 amd64 BigBlueButton WebRTC SFU
ii bigbluebutton 1:3.0-2 amd64 Open source web conferencing platform (bbb)
ii libopusenc0 0.2.1-1bbb2 amd64 High-level API for encoding Ogg Opus audio streams