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With support for multimodal models becoming more varied and common it is important for clients to be able to easily see what capabilities a model has. Retuning these from the show endpoint will allow clients to easily see what a model can do.

This reverts commit 7a0899d6.

- update chat docs
- add messages chat endpoint
- remove deprecated context and template generate parameters from docs
- context and template are still supported for the time being and will continue to work as expected
- add partial response to chat history

The `html/template` package automatically HTML-escapes interpolated strings in templates. This behavior is undesirable because it causes prompts like `<h1>hello` to be escaped to `&lt;h1&gt;hello` before being passed to the LLM.

The included test case passes, but before the code change, it failed:

```
--- FAIL: TestModelPrompt
    images_test.go:21: got "a&lt;h1&gt;b", want "a<h1>b"
```