Installer: Minor tweaks to UI

Installer: Minor tweaks to UI

- add a tooltip to the "help" links to make it clear they'll expand in place
- don't use an IMG tag for the download link;
  instead, apply a css background to the A element, so that on hover
  there isn't an underlined space between the link text and the image;
- grammar fix to sqlite help message: replace a colon with a semicolon.

Change-Id: Iacf5ed57d983a21d87467c848823607032901cec