Just like it happened with proposals, the button to select/deselect an
investment wasn't very intuitive; for example, it wasn't obvious that
pressing a button saying "selected" would deselect the investment.
So we're using a switch control, like we do to enable/disable features
since commit fabe97e50.
Note that we're making the text of the switch smaller than in other
places because the text in the investments table it is also smaller
(we're using `font-size: inherit` for that purpose). That made the
button look weird because we were using rems instead of ems for the
width of the button, so we're adjusting that as well.
Also note we're changing the width of the switch to `6em` instead of
`6.25em` (which would be 100px if 1em is 16px). We're doing so because
we used 100 for the minimum width because it's a round number, so
now we're using another round number.
