[web] Fix See more... button alignment regression

Summary:
Noticed a regression in the See more... sidebar button's alignment with the sidebars. Fixed this by adjusting the padding-left.

As an aside, because the ChatThreadList component is being actively worked on, I suspect the alignment may be messed up in the future if the arrow/sidebar get bigger or smaller. Therefore, we should probably pull in the padding from some central place so we don't introduce new regressions when modifying the styling of one part of the component.

Test Plan:
Tested on Chrome/Safari and works as expected:
Before:

Reviewers: atul, benschac

Reviewed By: atul

Subscribers: benschac, ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3917