Add modpack installer to Start Screen

[jwrober]

Is your feature request related to a problem? Please describe.
I am not really sure how many people know about the modpack installer as a way to download added content from our third party set of developers. I think it would be best to promote this more.

Describe the solution you'd like
Add a "Add Ons" or "Mods" or some other such button to the main start screen. This button will bring up the modpack installer allowing the user to pick what they want.

Describe alternatives you've considered
None really. Obviously leaving it alone or putting some text there is another option, but button seems the best solution.

Additional context
I was reviewing our docs repo and thought of it while reading the page about using the modpack installer.

[jwrober]

As a further note: We have discussed adding the ability to download add-ons via the mp installer in the game interface as well, this issue is not to override that option.