Make welcome page mobile friendly

[WesleyBranton]

The landing page that is displayed when first installing the add-on is not mobile friendly. This should be completed before the add-on is made available on Firefox for Android.

Since this work must be completed on the web-server, only I can complete this issue. I have created a GitHub issue in the private repo for my web-server.

This potentially also applies to the uninstall survey and update information pages.