tweak: Margins and dapp names

[dhedey]

(Replaces #304 which was created against my fork before I had write perms, but that meant the tests couldn't run)

Overview

Fixes the couple of issues from #303 - notably:

• If someone doesn't provide a dAppName, I use dApp so it doesn't look weird
• I've reduced the superfluous lower padding on the Requests tab (which was coming from a few sources)
• The blurred edge mask to hint that there's a scroll bar still:
  • Still exists, and now has a fix height of 10px
  • Still isn't visible if scrolled fully to the bottom
  • Has been redefined slightly so that it doesn't need any overrides on different screens or screen sizes any more
• The max height of the content of the Sharing tab is larger, in line with the requests tab (it often requires I scroll at the moment, this should improve matters)

Testing

• I've manually gone through all the storybook on mobile/non-mobile mode

Before / After