This introduces a couple of new CSS modifier flags to

rb-c-review-request-field-tabular:


-is-loading: Prepares the field to appear blank except for a

  consumer-provided spinner or other loading indicator.


-is-vertical-on-mobile: Sets the field to show a row per cell for

  the table when in mobile mode.


The latter used to be the default, but now must be opt-in. It can also

be set by a specialization of the component in LessCSS using

#rb-ns-review-requests.fields.tabular.set-vertical-on-mobile()
To allow specializations to also modify behavior styling in mobile mode,

#rb-ns-review-requests.fields.tabular.on-mobile-mode() was also added.

This kicks mobile mode in a bit sooner (at a greater width) than the old

styles did.
There are also fixes for background colors when in mobile mode, and for

undoing any min-width/max-width settings from a specialization.
Currently, only the issue summary table uses this field. It's been

updated to explicitly turn on vertical mode, retaining existing

behavior.

Tested the new features with both the issue summary table and with an

in-development restyle of some fields. Everything worked and looked

correct on desktop and mobile.