Update the model selection section's text

### Description
Multiple small changes to the current model selection section:
- [x] Remove the row of text "Select a model:"
- [x] Remove "Experiment name"
- [x] Expand the input box for the model name to the same width of the dropdown box below
- [x] Add guide text in the model name input box "Name your model"
- [x] Update text from "Existing model" to "Select an existing model"

Reasoning: we currently have a flat level of experiment/model, thus removing the specific language to avoid confusion

See:
* screenshot below
* [Figma mockup](https://www.figma.com/file/kiqNACOut8sGePs8tr7W57/ML-plugin?type=design&node-id=1016%3A41732&mode=design&t=87xICb4LlLj4Lmof-1)

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### Size
S

### Scope
Please ignore other styling changes that are not mentioned

### Additional notes
V1 deliverable