This introduces the rb-c-content-header component, which provides a

consistent header above an area of content. This can be used for the

main page header (sitting in-between the page's header bar and the

content area) or above sections of forms, lists of items, etc.
The content area allows for a title and a list of actions, which are

placed immediately to the right of the title.
Headers currently support two section levels. The -is-main modifier

indicates the header is the main one for the page (a level 1 heading),

which gives it some standardized padding and a slightly bigger font

size. Otherwise, it's a level 2 heading, which doesn't have any

margins/padding by default and uses a smaller font size.
There's also a #rb-ns-ui.content-header.add-title-styles() mixin

function available that injects the color, font, and uppercase styles

into an element to give the title appearance, helping other components

(such as the sidebar) keep a consistent look.
Content headers can be put into a mobile mode by using

#rb-ns-ui.content-header.set-mobile-mode-max-width(). Any viewport

width less than or equal to the provided width will result in the header

being in mobile mode, which slightly increases the font size (to better

differentiate between it and any content below it -- since in mobile

mode the content box area will not have any border), and to adjust

padding appropriately.
This component is currently being used on datagrid pages (in place of

the older styles for the datagrid header), and specialized through a

rb-c-datagrid-header component (currently only used to alter padding

for the case of tabs above datagrids).
The new Administration UI will make use of this component in several

pages.

Tested that all datagrid pages are still looking and working fine in

desktop and mobile modes.
Tested that the sidebar still looks correct with the usage of

add-title-styles().
Tested the component with upcoming administration UI work.