Design `Create a Threat Model` Quickstart Process

[MatKallada]

This issue raises the concern about simplifying the model creating/loading process. I would like to remove the create and load pages and put them all into the main splash page.

Below is the current WIP for the intended new create a threat model page, which evades the load and create pages.

In my opinion, it's more smooth & simplified: load/create models all on a single "quickstart" page.

[Glavin001]

I like the new look!

Simplifying the creating/loading process is related to #54. Feel free to close #54 in favour of this more comprehensive issue.

[Glavin001]

By the way, you can create a Pull Request at any time, and conventionally screenshots, questions, comments, and other issues to review are discussed in the Pull Request's Issue.

[Frozenfire92]

The new design looks nice! I think adding a margin between the create/load buttons would look nice as well as left aligning the label text and moving it closer to the input boxes