Change alignment for table headings

Currently the vertical-alignment for tables is bottom, see https://github.com/twbs/bootstrap/blame/v3-dev/less/tables.less#L42

Issue created after discussion on https://github.com/xwiki/xwiki-platform/commit/c3afc8a5332fa493b14139e9f9c07e9afd468462 on Notifications

Related discussion https://ux.stackexchange.com/questions/110135/table-headers-uppercase-or-lowercase-left-or-center-aligned-top-or-bottom