Save threat model to local storage

[Glavin001]

[Easy] Since this application is entirely client-side, we can make use of local storage to save and load threats.

[Glavin001]

@curtisko mentioned that this should include File API.

[Glavin001]

Blocked by #22. We require the file format structure (serialized and deserialized form of threat model data) before we can write it to LocalStorage or a File.

[MatKallada]

Not necessarily blocked by #22; could be blocked either direction. In fact, first implementing local storage saving would be more logical.

• [x] Create saving data threat model format
• [x] Save to local storage feature
• [x] Discuss filename, since it appears to be a concern

[Glavin001]

I considered #22 to include the task of "Create saving data threat model format", as we could write Unit Tests for importing our threat model file ( #22 ) with an example data using our designed threat model format and not yet require the completed implementation of this issue (save that file to local storage).