Export layers from the navigator to MS Excel format (.xlsx)

[isaisabel]

As a user I want to be able to export the main Navigator view to MS Excel format.

[sarahyoder]

Outstanding pull request from my branch (https://github.com/mitre/attack-navigator/pull/48) resolves this issue.

The pull adds an additional function (and button) to the navigator UI.
When using the function, a excel document is created by taking in the attributes of the current layer (current tactics, defined coloring, and scoring) and recreates the information for output.

The excel document then downloads, inheriting the layer name.

Further work needs to be done on how the scoring coloring is being displayed, as well as working on the coloring precedence (i.e. item has both color and score).

[isaisabel]

Completed features and merged pull request #48 into develop.