Fix opacity of popover arrows.

Previously, the percentage of the LESS `fadein` function was being used directly as the opacity, which resulted in an opacity of `0.05` rather than `0.25` as the LESS version generates.

We can use the SASS `fade_in` function to more accurately generate the correct color using the same values; the only difference is that SASS's `fade_in` expects a decimal value, rather than the percentage that LESS accepts.

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When I ran the `rake convert` task, several other unrelated files were changed, which I haven't included in this commit because it's unclear which changes are a result of this fix, and which are a result of changes in the upstream Bootstrap LESS files. Please let me know if I should explicitly include generated SASS files in this pull request, and I'll get right on it.