[WIP] Upgrade to the latest version of sae_vis

[jordansauce]

Description

See issue #54: Currently our feature dashboards rely on an old commit of Callum McDougall's sae_vis library. Specifically, callummcdougall/sae_vis@b28a0f7.
This PR will make our code compatible with the latest version of sae_vis (0.2), which was significantly restructured in March 2024. The recent changes include:

• The ability to view multiple features on the same page (rather than just one feature at a time)
• D3-backed visualisations (which can do things like add lines to histograms as you hover over tokens)
• SAE-class agnostic functions parse_feature_data() and parse_prompt_data() are now added to sae_vis, so we can remove them from our codebase - see Added SAE class agnostic functions callummcdougall/sae_vis#17
• Significantly modified data storing classes and HTML generating functions

No more major restructures of sae_vis are planned from now (callummcdougall/sae_vis#15 (comment)) so this is a good time to upgrade.

However it also seems much slower to generate prompt-centric dashboards now (either that or I'm doing something wrong), so this shouldn't be merged until that's fixed.